### 4.12 Scripting

Scripts allow authors to add interactivity to their documents.

Authors are encouraged to use declarative alternatives to scripting where possible, as declarative mechanisms are often more maintainable, and many users disable scripting.

For example, instead of using script to show or hide a section to show more details, the `details` element could be used.

Authors are also encouraged to make their applications degrade gracefully in the absence of scripting support.

For example, if an author provides a link in a table header to dynamically resort the table, the link could also be made to function without scripts by requesting the sorted table from the server.

#### 4.12.1 The `script` element

Categories:
  Metadata content.
  Flow content.
  Phrasing content.
  Script-supporting element.
Contexts in which this element can be used:
  Where metadata content is expected.
  Where phrasing content is expected.
  Where script-supporting elements are expected.
Content model:
  If there is no `src` attribute, depends on the value of the `type` attribute, but must match script content restrictions.
  If there is a `src` attribute, the element must be either empty or contain only script documentation that also matches script content restrictions.
Tag omission in text/html:
  Neither tag is omissible.
Content attributes:
  Global attributes
  `src` — Address of the resource
  `type` — Type of embedded resource
  `charset` — Character encoding of the external script resource
  `async` — Execute script when available, without blocking
  `defer` — Defer script execution
  `crossorigin` — How the element handles crossorigin requests
DOM interface:

      interface HTMLScriptElement : HTMLElement {
        attribute DOMString src;
        attribute DOMString type;
        attribute DOMString charset;
        attribute boolean async;
        attribute boolean defer;
        attribute DOMString? crossOrigin;
        attribute DOMString text;
      };

The `script` element allows authors to include dynamic script and data blocks in their documents. The element does not represent content for the user.

When used to include dynamic scripts, the scripts may either be embedded inline or may be imported from an external file using the `src` attribute. If the language is not that described by "`text/javascript`", then the `type` attribute must be present, as described below. Whatever language is used, the contents of the `script` element must conform with the requirements of that language's specification.

When used to include data blocks (as opposed to scripts), the data must be embedded inline, the format of the data must be given using the `type` attribute, the `src` attribute must not be specified, and the contents of the `script` element must conform to the requirements defined for the format used.

The `type` attribute gives the language of the script or format of the data. If the attribute is present, its value must be a valid MIME type. The `charset` parameter must not be specified. The default, which is used if the attribute is absent, is "`text/javascript`".

The `src` attribute, if specified, gives the address of the external script resource to use. The value of the attribute must be a valid non-empty URL potentially surrounded by spaces identifying a script resource of the type given by the `type` attribute, if the attribute is present, or of the type "`text/javascript`", if the attribute is absent. A resource is a script resource of a given type if that type identifies a scripting language and the resource conforms with the requirements of that language's specification.

The `charset` attribute gives the character encoding of the external script resource. The attribute must not be specified if the `src` attribute is not present. If the attribute is set, its value must be an ASCII case-insensitive match for one of the labels of an encoding, and must specify the same encoding as the `charset` parameter of the Content-Type metadata of the external file, if any. [ENCODING]

The `async` and `defer` attributes are boolean attributes that indicate how the script should be executed. The `defer` and `async` attributes must not be specified if the `src` attribute is not present.

There are three possible modes that can be selected using these attributes. If the `async` attribute is present, then the script will be executed as soon as it is available, but without blocking further parsing of the page. If the `async` attribute is not present but the `defer` attribute is present, then the script is executed when the page has finished parsing. If neither attribute is present, then the script is fetched and executed immediately, before the user agent continues parsing the page.

The exact processing details for these attributes are, for mostly historical reasons, somewhat non-trivial, involving a number of aspects of HTML. The implementation requirements are therefore by necessity scattered throughout the specification. The algorithms below (in this section) describe the core of this processing, but these algorithms reference and are referenced by the parsing rules for `script` start and end tags in HTML, in foreign content, and in XML, the rules for the `document.write()` method, the handling of scripting, etc.

The `defer` attribute may be specified even if the `async` attribute is specified, to cause legacy Web browsers that only support `defer` (and not `async`) to fall back to the `defer` behaviour instead of the blocking behaviour that is the default.

The `crossorigin` attribute is a CORS settings attribute. It controls, for scripts that are obtained from other origins, whether error information will be exposed.

Changing the `src`, `type`, `charset`, `async`, `defer`, and `crossorigin` attributes dynamically has no direct effect; these attribute are only used at specific times described below.

A `script` element has several associated pieces of state.

The first is a flag indicating whether or not the script block has been "already started". Initially, `script` elements must have this flag unset (script blocks, when created, are not "already started"). The cloning steps for `script` elements must set the "already started" flag on the copy if it is set on the element being cloned.

The second is a flag indicating whether the element was "parser-inserted". Initially, `script` elements must have this flag unset. It is set by the HTML parser and the XML parser on `script` elements they insert and affects the processing of those elements.

The third is a flag indicating whether the element will "non-blocking". Initially, `script` elements must have this flag set. It is unset by the HTML parser and the XML parser on `script` elements they insert. In addition, whenever a `script` element whose "non-blocking" flag is set has a `async` content attribute added, the element's "non-blocking" flag must be unset.

The fourth is a flag indicating whether or not the script block is "ready to be parser-executed". Initially, `script` elements must have this flag unset (script blocks, when created, are not "ready to be parser-executed"). This flag is used only for elements that are also "parser-inserted", to let the parser know when to execute the script.

The last few pieces of state are the script block's type, the script block's character encoding, and the script block's fallback character encoding. They are determined when the script is prepared, based on the attributes on the element at that time, and the `script` element's node document.

When a `script` element that is not marked as being "parser-inserted" experiences one of the events listed in the following list, the user agent must immediately prepare the `script` element:

- The `script` element gets inserted into a document, at the time the node is inserted according to the DOM, after any other `script` elements inserted at the same time that are earlier in the `Document` in tree order.
- The `script` element is in a `Document` and a node or document fragment is inserted into the `script` element, after any `script` elements inserted at that time.
- The `script` element is in a `Document` and has a `src` attribute set where previously the element had no such attribute.

To prepare a script, the user agent must act as follows:

1. If the `script` element is marked as having "already started", then the user agent must abort these steps at this point. The script is not executed.

2. If the element has its "parser-inserted" flag set, then set was-parser-inserted to true and unset the element's "parser-inserted" flag. Otherwise, set was-parser-inserted to false.

   This is done so that if parser-inserted `script` elements fail to run when the parser tries to run them, e.g. because they are empty or specify an unsupported scripting language, another script can later mutate them and cause them to run again.

3. If was-parser-inserted is true and the element does not have an `async` attribute, then set the element's "non-blocking" flag to true.

   This is done so that if a parser-inserted `script` element fails to run when the parser tries to run it, but it is later executed after a script dynamically updates it, it will execute in a non-blocking fashion even if the `async` attribute isn't set.

4. If the element has no `src` attribute, and its child nodes, if any, consist only of comment nodes and empty `Text` nodes, then the user agent must abort these steps at this point. The script is not executed.

5. If the element is not in a `Document`, then the user agent must abort these steps at this point. The script is not executed.

6. If either:

   - the `script` element has a `type` attribute and its value is the empty string, or
   - the `script` element has no `type` attribute but it has a `language` attribute and that attribute's value is the empty string, or
   - the `script` element has neither a `type` attribute nor a `language` attribute, then

   ...let the script block's type for this `script` element be "`text/javascript`".

   Otherwise, if the `script` element has a `type` attribute, let the script block's type for this `script` element be the value of that attribute with any leading or trailing sequences of space characters removed.

   Otherwise, the element has a non-empty `language` attribute; let the script block's type for this `script` element be the concatenation of the string "`text/`" followed by the value of the `language` attribute.

   The `language` attribute is never conforming, and is always ignored if there is a `type` attribute present.

7. If the user agent does not support the scripting language given by the script block's type for this `script` element, then the user agent must abort these steps at this point. The script is not executed.

8. If was-parser-inserted is true, then flag the element as "parser-inserted" again, and set the element's "non-blocking" flag to false.

9. The user agent must set the element's "already started" flag.

   The state of the element at this moment is later used to determine the script source.

10. If the element is flagged as "parser-inserted", but the element's node document is not the `Document` of the parser that created the element, then abort these steps.

11. If scripting is disabled for the `script` element, then the user agent must abort these steps at this point. The script is not executed.

    The definition of scripting is disabled means that, amongst others, the following scripts will not execute: scripts in `XMLHttpRequest`'s `responseXML` documents, scripts in `DOMParser`-created documents, scripts in documents created by `XSLTProcessor`'s `transformToDocument` feature, and scripts that are first inserted by a script into a `Document` that was created using the `createDocument()` API. [XHR] [DOMPARSING] [DOM]

12. If the `script` element has an `event` attribute and a `for` attribute, then run these substeps:

    1. Let for be the value of the `for` attribute.

    2. Let event be the value of the `event` attribute.

    3. Strip leading and trailing whitespace from event and for.

    4. If for is not an ASCII case-insensitive match for the string "`window`", then the user agent must abort these steps at this point. The script is not executed.

    5. If event is not an ASCII case-insensitive match for either the string "`onload`" or the string "`onload()`", then the user agent must abort these steps at this point. The script is not executed.

13. If the `script` element has a `charset` attribute, then let the script block's character encoding for this `script` element be the result of getting an encoding from the value of the `charset` attribute.

    Otherwise, let the script block's fallback character encoding for this `script` element be the same as the encoding of the document itself.

    Only one of these two pieces of state is set.

14. If the element has a `src` content attribute, run these substeps:

    1. Let src be the value of the element's `src` attribute.

    2. If src is the empty string, queue a task to fire a simple event named `error` at the element, and abort these steps.

    3. Resolve src relative to the element.

    4. If the previous step failed, queue a task to fire a simple event named `error` at the element, and abort these steps.

    5. Do a potentially CORS-enabled fetch of the resulting absolute URL, with the mode being the current state of the element's `crossorigin` content attribute, the origin being the origin of the `script` element's node document, and the default origin behaviour set to taint.

       The resource obtained in this fashion can be either CORS-same-origin or CORS-cross-origin. This only affects how error reporting happens.

       For performance reasons, user agents may start fetching the script (as defined above) as soon as the `src` attribute is set, instead, in the hope that the element will be inserted into the document (and that the `crossorigin` attribute won't change value in the meantime). Either way, once the element is inserted into the document, the load must have started as described in this step. If the UA performs such prefetching, but the element is never inserted in the document, or the `src` attribute is dynamically changed, or the `crossorigin` attribute is dynamically changed, then the user agent will not execute the script so obtained, and the fetching process will have been effectively wasted.

15. Then, the first of the following options that describes the situation must be followed:

    If the element has a `src` attribute, and the element has a `defer` attribute, and the element has been flagged as "parser-inserted", and the element does not have an `async` attribute

      The element must be added to the end of the list of scripts that will execute when the document has finished parsing associated with the `Document` of the parser that created the element.

      The task that the networking task source places on the task queue once the fetching algorithm has completed must set the element's "ready to be parser-executed" flag. The parser will handle executing the script.

    If the element has a `src` attribute, and the element has been flagged as "parser-inserted", and the element does not have an `async` attribute

      The element is the pending parsing-blocking script of the `Document` of the parser that created the element. (There can only be one such script per `Document` at a time.)

      The task that the networking task source places on the task queue once the fetching algorithm has completed must set the element's "ready to be parser-executed" flag. The parser will handle executing the script.

    If the element does not have a `src` attribute, and the element has been flagged as "parser-inserted", and either the parser that created the `script` is an XML parser or it's an HTML parser whose script nesting level is not greater than one, and the `Document` of the HTML parser or XML parser that created the `script` element has a style sheet that is blocking scripts

      The element is the pending parsing-blocking script of the `Document` of the parser that created the element. (There can only be one such script per `Document` at a time.)

      Set the element's "ready to be parser-executed" flag. The parser will handle executing the script.

    If the element has a `src` attribute, does not have an `async` attribute, and does not have the "non-blocking" flag set

      The element must be added to the end of the list of scripts that will execute in order as soon as possible associated with the node document of the `script` element at the time the prepare a script algorithm started.

      The task that the networking task source places on the task queue once the fetching algorithm has completed must run the following steps:

      1. If the element is not now the first element in the list of scripts that will execute in order as soon as possible to which it was added above, then mark the element as ready but abort these steps without executing the script yet.

      2. Execution: Execute the script block corresponding to the first script element in this list of scripts that will execute in order as soon as possible.

      3. Remove the first element from this list of scripts that will execute in order as soon as possible.

      4. If this list of scripts that will execute in order as soon as possible is still not empty and the first entry has already been marked as ready, then jump back to the step labeled execution.

    If the element has a `src` attribute

      The element must be added to the set of scripts that will execute as soon as possible of the node document of the `script` element at the time the prepare a script algorithm started.

      The task that the networking task source places on the task queue once the fetching algorithm has completed must execute the script block and then remove the element from the set of scripts that will execute as soon as possible.

    Otherwise
      The user agent must immediately execute the script block, even if other scripts are already executing.

Fetching an external script must delay the load event of the element's node document until the task that is queued by the networking task source once the resource has been fetched (defined above) has been run.

The pending parsing-blocking script of a `Document` is used by the `Document`'s parser(s).

If a `script` element that blocks a parser gets moved to another `Document` before it would normally have stopped blocking that parser, it nonetheless continues blocking that parser until the condition that causes it to be blocking the parser no longer applies (e.g. if the script is a pending parsing-blocking script because there was a style sheet that is blocking scripts when it was parsed, but then the script is moved to another `Document` before the style sheet loads, the script still blocks the parser until the style sheets are all loaded, at which time the script executes and the parser is unblocked).

When the user agent is required to execute a script block, it must run the following steps:

1. If the element is flagged as "parser-inserted", but the element's node document is not the `Document` of the parser that created the element, then abort these steps.

2. Jump to the appropriate set of steps from the list below:

   If the load resulted in an error (for example a DNS error, or an HTTP 404 error)

     Executing the script block must just consist of firing a simple event named `error` at the element.

   If the load was successful

     Executing the script block must consist of running the following steps. For the purposes of these steps, the script is considered to be from an external file if, while the prepare a script algorithm above was running for this script, the `script` element had a `src` attribute specified.

     1. Initialise the script block's source as follows:

        If the script is from an external file and the script block's type is a text-based language

          The contents of that file, interpreted as a Unicode string, are the script source.

          To obtain the Unicode string, the user agent run the following steps:

          1. If the resource's Content Type metadata, if any, specifies a character encoding, and the user agent supports that encoding, then let character encoding be that encoding, and jump to the bottom step in this series of steps.

          2. If the algorithm above set the script block's character encoding, then let character encoding be that encoding, and jump to the bottom step in this series of steps.

          3. Let character encoding be the script block's fallback character encoding.

          4. If the specification for the script block's type gives specific rules for decoding files in that format to Unicode, follow them, using character encoding as the character encoding specified by higher-level protocols, if necessary.

             Otherwise, decode the file to Unicode, using character encoding as the fallback encoding.

             The decode algorithm overrides character encoding if the file contains a BOM.

        If the script is from an external file and the script block's type is an XML-based language

          The external file is the script source. When it is later executed, it must be interpreted in a manner consistent with the specification defining the language given by the script block's type.

        If the script is inline and the script block's type is a text-based language

          The value of the `text` IDL attribute at the time the element's "already started" flag was last set is the script source.

        If the script is inline and the script block's type is an XML-based language

          The child nodes of the `script` element at the time the element's "already started" flag was last set are the script source.

     2. Fire a simple event named `beforescriptexecute` that bubbles and is cancelable at the `script` element.

        If the event is canceled, then abort these steps.

     3. If the script is from an external file, then increment the ignore-destructive-writes counter of the `script` element's node document. Let neutralised doc be that `Document`.

     4. Let old script element be the value to which the `script` element's node document's `currentScript` object was most recently initialised.

     5. Initialise the `script` element's node document's `currentScript` object to the `script` element.

     6. Create a script, using the script block's source, the URL from which the script was obtained, the script block's type as the scripting language, and the environment settings object of the `script` element's node document's `Window` object.

        If the script came from a resource that was fetched in the steps above, and the resource was CORS-cross-origin, then pass the muted errors flag to the create a script algorithm as well.

        This is where the script is compiled and actually executed.

     7. Initialise the `script` element's node document's `currentScript` object to old script element.

     8. Decrement the ignore-destructive-writes counter of neutralised doc, if it was incremented in the earlier step.

     9. Fire a simple event named `afterscriptexecute` that bubbles (but is not cancelable) at the `script` element.

     10. If the script is from an external file, fire a simple event named `load` at the `script` element.

         Otherwise, the script is internal; queue a task to fire a simple event named `load` at the `script` element.

The IDL attributes `src`, `type`, `charset`, `defer`, each must reflect the respective content attributes of the same name.

The `crossOrigin` IDL attribute must reflect the `crossorigin` content attribute.

The `async` IDL attribute controls whether the element will execute asynchronously or not. If the element's "non-blocking" flag is set, then, on getting, the `async` IDL attribute must return true, and on setting, the "non-blocking" flag must first be unset, and then the content attribute must be removed if the IDL attribute's new value is false, and must be set to the empty string if the IDL attribute's new value is true. If the element's "non-blocking" flag is not set, the IDL attribute must reflect the `async` content attribute.

script . `text` [ = value ]

  Returns the contents of the element, ignoring child nodes that aren't `Text` nodes.

  Can be set, to replace the element's children with the given value.

The IDL attribute `text` must return a concatenation of the contents of all the `Text` nodes that are children of the `script` element (ignoring any other nodes such as comments or elements), in tree order. On setting, it must act the same way as the `textContent` IDL attribute.

When inserted using the `document.write()` method, `script` elements execute (typically blocking further script execution or HTML parsing), but when inserted using `innerHTML` and `outerHTML` attributes, they do not execute at all.

In this example, two `script` elements are used. One embeds an external script, and the other includes some data.

    <script src="game-engine.js"></script>
    <script type="text/x-game-map">
    ........U.........e
    o............A....e
    .....A.....AAA....e
    .A..AAA...AAAAA...e
    </script>

The data in this case might be used by the script to generate the map of a video game. The data doesn't have to be used that way, though; maybe the map data is actually embedded in other parts of the page's markup, and the data block here is just used by the site's search engine to help users who are looking for particular features in their game maps.

The following sample shows how a script element can be used to define a function that is then used by other parts of the document. It also shows how a `script` element can be used to invoke script while the document is being parsed, in this case to initialise the form's output.

    <script>
     function calculate(form) {
       var price = 52000;
       if (form.elements.brakes.checked)
         price += 1000;
       if (form.elements.radio.checked)
         price += 2500;
       if (form.elements.turbo.checked)
         price += 5000;
       if (form.elements.sticker.checked)
         price += 250;
       form.elements.result.value = price;
     }
    </script>
    <form name="pricecalc" onsubmit="return false" onchange="calculate(this)">
     <fieldset>
      <legend>Work out the price of your car</legend>
      <p>Base cost: £52000.</p>
      <p>Select additional options:</p>
      <ul>
       <li><label><input type=checkbox name=brakes> Ceramic brakes (£1000)</label></li>
       <li><label><input type=checkbox name=radio> Satellite radio (£2500)</label></li>
       <li><label><input type=checkbox name=turbo> Turbo charger (£5000)</label></li>
       <li><label><input type=checkbox name=sticker> "XZ" sticker (£250)</label></li>
      </ul>
      <p>Total: £<output name=result></output></p>
     </fieldset>
     <script>
      calculate(document.forms.pricecalc);
     </script>
    </form>

##### 4.12.1.1 Scripting languages

A user agent is said to support the scripting language if each component of the script block's type is an ASCII case-insensitive match for the corresponding component in the MIME type string of a scripting language that the user agent implements.

The following lists the MIME type strings that user agents must recognize, and the languages to which they refer:

`application/ecmascript`
`application/javascript`
`application/x-ecmascript`
`application/x-javascript`
`text/ecmascript`
`text/javascript`
`text/javascript1.0`
`text/javascript1.1`
`text/javascript1.2`
`text/javascript1.3`
`text/javascript1.4`
`text/javascript1.5`
`text/jscript`
`text/livescript`
`text/x-ecmascript`
`text/x-javascript`
  JavaScript. [ECMA262]

User agents may support other MIME types for other languages, but must not support other MIME types for the languages in the list above. User agents are not required to support the languages listed above.

The following MIME types (with or without parameters) must not be interpreted as scripting languages:

- `text/plain`
- `text/xml`
- `application/octet-stream`
- `application/xml`

These types are explicitly listed here because they are poorly-defined types that are nonetheless likely to be used as formats for data blocks, and it would be problematic if they were suddenly to be interpreted as script by a user agent.

When examining types to determine if they represent supported languages, user agents must not ignore MIME parameters. Types are to be compared including all parameters.

For example, types that include the `charset` parameter will not be recognised as referencing any of the scripting languages listed above.

##### 4.12.1.2 Restrictions for contents of `script` elements

The easiest and safest way to avoid the rather strange restrictions described in this section is to always escape "`<!--`" as "`<\!--`", "`<script`" as "`<\script`", and "`</script`" as "`<\/script`" when these sequences appear in literals in scripts (e.g. in strings, regular expressions, or comments), and to avoid writing code that uses such constructs in expressions. Doing so avoids the pitfalls that the restrictions in this section are prone to triggering: namely, that, for historical reasons, parsing of `script` blocks in HTML is a strange and exotic practice that acts unintuitively in the face of these sequences.

The `textContent` of a `script` element must match the `script` production in the following ABNF, the character set for which is Unicode. [ABNF]

    script        = outer *( comment-open inner comment-close outer )

    outer         = < any string that doesn't contain a substring that matches not-in-outer >
    not-in-outer  = comment-open
    inner         = < any string that doesn't contain a substring that matches not-in-inner >
    not-in-inner  = comment-close / script-open

    comment-open  = "<!--"
    comment-close = "-->"
    script-open   = "<" s c r i p t tag-end

    s             =  %x0053 ; U+0053 LATIN CAPITAL LETTER S
    s             =/ %x0073 ; U+0073 LATIN SMALL LETTER S
    c             =  %x0043 ; U+0043 LATIN CAPITAL LETTER C
    c             =/ %x0063 ; U+0063 LATIN SMALL LETTER C
    r             =  %x0052 ; U+0052 LATIN CAPITAL LETTER R
    r             =/ %x0072 ; U+0072 LATIN SMALL LETTER R
    i             =  %x0049 ; U+0049 LATIN CAPITAL LETTER I
    i             =/ %x0069 ; U+0069 LATIN SMALL LETTER I
    p             =  %x0050 ; U+0050 LATIN CAPITAL LETTER P
    p             =/ %x0070 ; U+0070 LATIN SMALL LETTER P
    t             =  %x0054 ; U+0054 LATIN CAPITAL LETTER T
    t             =/ %x0074 ; U+0074 LATIN SMALL LETTER T

    tag-end       =  %x0009 ; U+0009 CHARACTER TABULATION (tab)
    tag-end       =/ %x000A ; U+000A LINE FEED (LF)
    tag-end       =/ %x000C ; U+000C FORM FEED (FF)
    tag-end       =/ %x0020 ; U+0020 SPACE
    tag-end       =/ %x002F ; U+002F SOLIDUS (/)
    tag-end       =/ %x003E ; U+003E GREATER-THAN SIGN (>)

When a `script` element contains script documentation, there are further restrictions on the contents of the element, as described in the section below.

The following script illustrates this issue. Suppose you have a script that contains a string, as in:

    var example = 'Consider this string: <!-- <script>';
    console.log(example);

If one were to put this string directly in a `script` block, it would violate the restrictions above:

    <script>
      var example = 'Consider this string: <!-- <script>';
      console.log(example);
    </script>

The bigger problem, though, and the reason why it would violate those restrictions, is that actually the script would get parsed weirdly: the script block above is not terminated. That is, what looks like a "`</script>`" end tag in this snippet is actually still part of the `script` block. The script doesn't execute (since it's not terminated); if it somehow were to execute, as it might if the markup looked as follows, it would fail because the script (highlighted here) is not valid JavaScript:

    <script>
      var example = 'Consider this string: <!-- <script>';
      console.log(example);
    </script>
    <!-- despite appearances, this is actually part of the script still! -->
    <script>
     ... // this is the same script block still...
    </script>

What is going on here is that for legacy reasons, "`<!--`" and "`<script`" strings in `script` elements in HTML need to be balanced in order for the parser to consider closing the block.

By escaping the problematic strings as mentioned at the top of this section, the problem is avoided entirely:

    <script>
      var example = 'Consider this string: <\!-- <\script>';
      console.log(example);
    </script>
    <!-- this is just a comment between script blocks -->
    <script>
     ... // this is a new script block
    </script>

It is possible for these sequences to naturally occur in script expressions, as in the following examples:

    if (x<!--y) { ... }
    if ( player<script ) { ... }

In such cases the characters cannot be escaped, but the expressions can be rewritten so that the sequences don't occur, as in:

    if (x < !--y) { ... }
    if (!--y > x) { ... }
    if (!(--y) > x) { ... }
    if (player < script) { ... }
    if (script > player) { ... }

Doing this also avoids a different pitfall as well: for related historical reasons, the string "<!--" in JavaScript is actually treated as a line comment start, just like "//".

##### 4.12.1.3 Inline documentation for external scripts

If a `script` element's `src` attribute is specified, then the contents of the `script` element, if any, must be such that the value of the `text` IDL attribute, which is derived from the element's contents, matches the `documentation` production in the following ABNF, the character set for which is Unicode. [ABNF]

    documentation = *( *( space / tab / comment ) [ line-comment ] newline )
    comment       = slash star *( not-star / star not-slash ) 1*star slash
    line-comment  = slash slash *not-newline

    ; characters
    tab           = %x0009 ; U+0009 CHARACTER TABULATION (tab)
    newline       = %x000A ; U+000A LINE FEED (LF)
    space         = %x0020 ; U+0020 SPACE
    star          = %x002A ; U+002A ASTERISK (*)
    slash         = %x002F ; U+002F SOLIDUS (/)
    not-newline   = %x0000-0009 / %x000B-10FFFF
                    ; a Unicode character other than U+000A LINE FEED (LF)
    not-star      = %x0000-0029 / %x002B-10FFFF
                    ; a Unicode character other than U+002A ASTERISK (*)
    not-slash     = %x0000-002E / %x0030-10FFFF
                    ; a Unicode character other than U+002F SOLIDUS (/)

This corresponds to putting the contents of the element in JavaScript comments.

This requirement is in addition to the earlier restrictions on the syntax of contents of `script` elements.

This allows authors to include documentation, such as license information or API information, inside their documents while still referring to external script files. The syntax is constrained so that authors don't accidentally include what looks like valid script while also providing a `src` attribute.

    <script src="cool-effects.js">
     // create new instances using:
     //    var e = new Effect();
     // start the effect using .play, stop using .stop:
     //    e.play();
     //    e.stop();
    </script>

##### 4.12.1.4 Interaction of `script` elements and XSLT

This section is non-normative.

This specification does not define how XSLT interacts with the `script` element. However, in the absence of another specification actually defining this, here are some guidelines for implementors, based on existing implementations:

- When an XSLT transformation program is triggered by an `<?xml-stylesheet?>` processing instruction and the browser implements a direct-to-DOM transformation, `script` elements created by the XSLT processor need to be marked "parser-inserted" and run in document order (modulo scripts marked `defer` or `async`), immediately, as the transformation is occurring.

- The `XSLTProcessor.transformToDocument()` method adds elements to a `Document` that is not in a browsing context, and, accordingly, any `script` elements they create need to have their "already started" flag set in the prepare a script algorithm and never get executed (scripting is disabled). Such `script` elements still need to be marked "parser-inserted", though, such that their `async` IDL attribute will return false in the absence of an `async` content attribute.

- The `XSLTProcessor.transformToFragment()` method needs to create a fragment that is equivalent to one built manually by creating the elements using `document.createElementNS()`. For instance, it needs to create `script` elements that aren't "parser-inserted" and that don't have their "already started" flag set, so that they will execute when the fragment is inserted into a document.

The main distinction between the first two cases and the last case is that the first two operate on `Document`s and the last operates on a fragment.

#### 4.12.2 The `noscript` element

Categories:
  Metadata content.
  Flow content.
  Phrasing content.
Contexts in which this element can be used:
  In a `head` element of an HTML document, if there are no ancestor `noscript` elements.
  Where phrasing content is expected in HTML documents, if there are no ancestor `noscript` elements.
Content model:
  When scripting is disabled, in a `head` element: in any order, zero or more `link` elements, zero or more `style` elements, and zero or more `meta` elements.
  When scripting is disabled, not in a `head` element: transparent, but there must be no `noscript` element descendants.
  Otherwise: text that conforms to the requirements given in the prose.
Tag omission in text/html:
  Neither tag is omissible.
Content attributes:
  Global attributes
DOM interface:
  Uses `HTMLElement`.

The `noscript` element represents nothing if scripting is enabled, and represents its children if scripting is disabled. It is used to present different markup to user agents that support scripting and those that don't support scripting, by affecting how the document is parsed.

When used in HTML documents, the allowed content model is as follows:

In a `head` element, if scripting is disabled for the `noscript` element

  The `noscript` element must contain only `link`, `style`, and `meta` elements.

In a `head` element, if scripting is enabled for the `noscript` element

  The `noscript` element must contain only text, except that invoking the HTML fragment parsing algorithm with the `noscript` element as the context element and the text contents as the input must result in a list of nodes that consists only of `link`, `style`, and `meta` elements that would be conforming if they were children of the `noscript` element, and no parse errors.

Outside of `head` elements, if scripting is disabled for the `noscript` element

  The `noscript` element's content model is transparent, with the additional restriction that a `noscript` element must not have a `noscript` element as an ancestor (that is, `noscript` can't be nested).

Outside of `head` elements, if scripting is enabled for the `noscript` element

  The `noscript` element must contain only text, except that the text must be such that running the following algorithm results in a conforming document with no `noscript` elements and no `script` elements, and such that no step in the algorithm throws an exception or causes an HTML parser to flag a parse error:

  1. Remove every `script` element from the document.
  2. Make a list of every `noscript` element in the document. For every `noscript` element in that list, perform the following steps:
     1. Let s be the concatenation of all the `Text` node children of the `noscript` element.
     2. Set the `outerHTML` attribute of the `noscript` element to the value of s. (This, as a side-effect, causes the `noscript` element to be removed from the document.) [DOMPARSING]

All these contortions are required because, for historical reasons, the `noscript` element is handled differently by the HTML parser based on whether scripting was enabled or not when the parser was invoked.

The `noscript` element must not be used in XML documents.

The `noscript` element is only effective in the HTML syntax, it has no effect in the XHTML syntax. This is because the way it works is by essentially "turning off" the parser when scripts are enabled, so that the contents of the element are treated as pure text and not as real elements. XML does not define a mechanism by which to do this.

The `noscript` element has no other requirements. In particular, children of the `noscript` element are not exempt from form submission, scripting, and so forth, even when scripting is enabled for the element.

In the following example, a `noscript` element is used to provide fallback for a script.

    <form action="calcSquare.php">
     <p>
      <label for=x>Number</label>:
      <input id="x" name="x" type="number">
     </p>
     <script>
      var x = document.getElementById('x');
      var output = document.createElement('p');
      output.textContent = 'Type a number; it will be squared right then!';
      x.form.appendChild(output);
      x.form.onsubmit = function () { return false; }
      x.oninput = function () {
        var v = x.valueAsNumber;
        output.textContent = v + ' squared is ' + v * v;
      };
     </script>
     <noscript>
      <input type=submit value="Calculate Square">
     </noscript>
    </form>

When script is disabled, a button appears to do the calculation on the server side. When script is enabled, the value is computed on-the-fly instead.

The `noscript` element is a blunt instrument. Sometimes, scripts might be enabled, but for some reason the page's script might fail. For this reason, it's generally better to avoid using `noscript`, and to instead design the script to change the page from being a scriptless page to a scripted page on the fly, as in the next example:

    <form action="calcSquare.php">
     <p>
      <label for=x>Number</label>:
      <input id="x" name="x" type="number">
     </p>
     <input id="submit" type=submit value="Calculate Square">
     <script>
      var x = document.getElementById('x');
      var output = document.createElement('p');
      output.textContent = 'Type a number; it will be squared right then!';
      x.form.appendChild(output);
      x.form.onsubmit = function () { return false; }
      x.oninput = function () {
        var v = x.valueAsNumber;
        output.textContent = v + ' squared is ' + v * v;
      };
      var submit = document.getElementById('submit');
      submit.parentNode.removeChild(submit);
     </script>
    </form>

The above technique is also useful in XHTML, since `noscript` is not supported in the XHTML syntax.

#### 4.12.3 The `template` element

Categories:
  Metadata content.
  Flow content.
  Phrasing content.
  Script-supporting element.
Contexts in which this element can be used:
  Where metadata content is expected.
  Where phrasing content is expected.
  Where script-supporting elements are expected.
  As a child of a `colgroup` element that doesn't have a `span` attribute.
Content model:
  Either: Metadata content.
  Or: Flow content.
  Or: The content model of `ol` and `ul` elements.
  Or: The content model of `dl` elements.
  Or: The content model of `figure` elements.
  Or: The content model of `ruby` elements.
  Or: The content model of `object` elements.
  Or: The content model of `video` and `audio` elements.
  Or: The content model of `table` elements.
  Or: The content model of `colgroup` elements.
  Or: The content model of `thead`, `tbody`, and `tfoot` elements.
  Or: The content model of `tr` elements.
  Or: The content model of `fieldset` elements.
  Or: The content model of `select` elements.
  Or: The content model of `details` elements.
  Or: The content model of `menu` elements whose `type` attribute is in the popup menu state.
Tag omission in text/html:
  Neither tag is omissible.
Content attributes:
  Global attributes
DOM interface:

      interface HTMLTemplateElement : HTMLElement {
        readonly attribute DocumentFragment content;
      };

The `template` element is used to declare fragments of HTML that can be cloned and inserted in the document by script.

In a rendering, the `template` element represents nothing.

template . `content`

  Returns the contents of the `template`, which are stored in a `DocumentFragment` associated with a different `Document` so as to avoid the `template` contents interfering with the main `Document`. (For example, this avoids form controls from being submitted, scripts from executing, and so forth.)

Each `template` element has an associated `DocumentFragment` object that is its template contents. When a `template` element is created, the user agent must run the following steps to establish the template contents:

1. Let doc be the `template` element's node document's appropriate template contents owner document.

2. Create a `DocumentFragment` object whose node document is doc.

3. Set the `template` element's template contents to the newly created `DocumentFragment` object.

A `Document` doc's appropriate template contents owner document is the `Document` returned by the following algorithm:

1. If doc is not a `Document` created by this algorithm, run these substeps:

   1. If doc does not yet have an associated inert template document then run these substeps:

      1. Let new doc be a new `Document` (that does not have a browsing context). This is "a `Document` created by this algorithm" for the purposes of the step above.

      2. If doc is an HTML document, mark new doc as an HTML document also.

      3. Let doc's associated inert template document be new doc.

   2. Set doc to doc's associated inert template document.

   Each `Document` not created by this algorithm thus gets a single `Document` to act as its proxy for owning the template contents of all its `template` elements, so that they aren't in a browsing context and thus remain inert (e.g. scripts do not run). Meanwhile, `template` elements inside `Document` objects that are created by this algorithm just reuse the same `Document` owner for their contents.

2. Return doc.

When a `template` element's node document changes, the user agent must run the following steps:

1. Let doc be the `template` element's new node document's appropriate template contents owner document.

2. Adopt the `template` element's template contents (a `DocumentFragment` object) into doc.

The `content` IDL attribute must return the `template` element's template contents.

The cloning steps for a `template` element node being cloned to a copy copy must run the following steps:

1. If the clone children flag is not set in the calling clone algorithm, abort these steps.

2. Let copied contents be the result of cloning all the children of node's template contents, with document set to copy's template contents's node document, and with the clone children flag set.

3. Append copied contents to copy's template contents.

In this example, a script populates a table four-column with data from a data structure, using a `template` to provide the element structure instead of manually generating the structure from markup.

    <!DOCTYPE html>
    <title>Cat data</title>
    <script>
     // Data is hard-coded here, but could come from the server
     var data = [
       { name: 'Pillar', color: 'Ticked Tabby', sex: 'Female (neutered)', legs: 3 },
       { name: 'Hedral', color: 'Tuxedo', sex: 'Male (neutered)', legs: 4 },
     ];
    </script>
    <table>
     <thead>
      <tr>
       <th>Name <th>Colour <th>Sex <th>Legs
     <tbody>
      <template id="row">
       <tr><td><td><td><td>
      </template>
    </table>
    <script>
     var template = document.querySelector('#row');
     for (var i = 0; i < data.length; i += 1) {
       var cat = data[i];
       var clone = template.content.cloneNode(true);
       var cells = clone.querySelectorAll('td');
       cells[0].textContent = cat.name;
       cells[1].textContent = cat.color;
       cells[2].textContent = cat.sex;
       cells[3].textContent = cat.legs;
       template.parentNode.appendChild(clone);
     }
    </script>

This example uses `cloneNode()` on the `template`'s contents; it could equivalently have used `document.importNode()`, which does the same thing. The only difference between these two APIs is when the node document is updated: with `cloneNode()` it is updated when the nodes are appended with `appendChild()`, with `document.importNode()` it is updated when the nodes are cloned.

##### 4.12.3.1 Interaction of `template` elements with XSLT and XPath

This section is non-normative.

This specification does not define how XSLT and XPath interact with the `template` element. However, in the absence of another specification actually defining this, here are some guidelines for implementors, which are intended to be consistent with other processing described in this specification:

- An XSLT processor based on an XML parser that acts as described in this specification needs to act as if `template` elements contain as descendants their template contents for the purposes of the transform.

- An XSLT processor that outputs a DOM needs to ensure that nodes that would go into a `template` element are instead placed into the element's template contents.

- XPath evaluation using the XPath DOM API when applied to a `Document` parsed using the HTML parser or the XML parser described in this specification needs to ignore template contents.

#### 4.12.4 The `canvas` element

Categories:
  Flow content.
  Phrasing content.
  Embedded content.
  Palpable content.
Contexts in which this element can be used:
  Where embedded content is expected.
Content model:
  Transparent, but with no interactive content descendants except for `a` elements, `img` elements with `usemap` attributes, `button` elements, `input` elements whose `type` attribute are in the Checkbox or Radio Button states, `input` elements that are buttons, `select` elements with a `multiple` attribute or a display size greater than 1, sorting interface `th` elements, and elements that would not be interactive content except for having the `tabindex` attribute specified.
Tag omission in text/html:
  Neither tag is omissible.
Content attributes:
  Global attributes
  `width` — Horizontal dimension
  `height` — Vertical dimension
DOM interface:

      typedef (CanvasRenderingContext2D or WebGLRenderingContext) RenderingContext;

      interface HTMLCanvasElement : HTMLElement {
        attribute unsigned long width;
        attribute unsigned long height;

        RenderingContext? getContext(DOMString contextId, any... arguments);
        boolean probablySupportsContext(DOMString contextId, any... arguments);

        void setContext(RenderingContext context);
        CanvasProxy transferControlToProxy();

        DOMString toDataURL(optional DOMString type, any... arguments);
        void toBlob(FileCallback? _callback, optional DOMString type, any... arguments);
      };

The `canvas` element provides scripts with a resolution-dependent bitmap canvas, which can be used for rendering graphs, game graphics, art, or other visual images on the fly.

Authors should not use the `canvas` element in a document when a more suitable element is available. For example, it is inappropriate to use a `canvas` element to render a page heading: if the desired presentation of the heading is graphically intense, it should be marked up using appropriate elements (typically `h1`) and then styled using CSS and supporting technologies such as Web Components.

When authors use the `canvas` element, they must also provide content that, when presented to the user, conveys essentially the same function or purpose as the `canvas`' bitmap. This content may be placed as content of the `canvas` element. The contents of the `canvas` element, if any, are the element's fallback content.

In interactive visual media, if scripting is enabled for the `canvas` element, and if support for `canvas` elements has been enabled, the `canvas` element represents embedded content consisting of a dynamically created image, the element's bitmap.

In non-interactive, static, visual media, if the `canvas` element has been previously associated with a rendering context (e.g. if the page was viewed in an interactive visual medium and is now being printed, or if some script that ran during the page layout process painted on the element), then the `canvas` element represents embedded content with the element's current bitmap and size. Otherwise, the element represents its fallback content instead.

In non-visual media, and in visual media if scripting is disabled for the `canvas` element or if support for `canvas` elements has been disabled, the `canvas` element represents its fallback content instead.

When a `canvas` element represents embedded content, the user can still focus descendants of the `canvas` element (in the fallback content). When an element is focused, it is the target of keyboard interaction events (even though the element itself is not visible). This allows authors to make an interactive canvas keyboard-accessible: authors should have a one-to-one mapping of interactive regions to focusable areas in the fallback content. (Focus has no effect on mouse interaction events.) [DOMEVENTS]

An element whose nearest `canvas` element ancestor is being rendered and represents embedded content is an element that is being used as relevant canvas fallback content.

The `canvas` element has two attributes to control the size of the element's bitmap: `width` and `height`. These attributes, when specified, must have values that are valid non-negative integers. The rules for parsing non-negative integers must be used to obtain their numeric values. If an attribute is missing, or if parsing its value returns an error, then the default value must be used instead. The `width` attribute defaults to 300, and the `height` attribute defaults to 150.

The intrinsic dimensions of the `canvas` element when it represents embedded content are equal to the dimensions of the element's bitmap.

The user agent must use a square pixel density consisting of one pixel of image data per coordinate space unit for the bitmaps of a `canvas` and its rendering contexts.

A `canvas` element can be sized arbitrarily by a style sheet, its bitmap is then subject to the 'object-fit' CSS property. [CSSIMAGES]

The bitmaps of `canvas` elements, as well as some of the bitmaps of rendering contexts, such as those described in the section on the `CanvasRenderingContext2D` object below, have an origin-clean flag, which can be set to true or false. Initially, when the `canvas` element is created, its bitmap's origin-clean flag must be set to true.

A `canvas` bitmap can also have a hit region list, as described in the `CanvasRenderingContext2D` section below.

A `canvas` element can have a rendering context bound to it. Initially, it does not have a bound rendering context. To keep track of whether it has a rendering context or not, and what kind of rendering context it is, a `canvas` also has a canvas context mode, which is initially none but can be changed to either direct-2d, direct-webgl, indirect, or proxied by algorithms defined in this specification.

When its canvas context mode is none, a `canvas` element has no rendering context, and its bitmap must be fully transparent black with an intrinsic width equal to the numeric value of the element's `width` attribute and an intrinsic height equal to the numeric value of the element's `height` attribute, those values being interpreted in CSS pixels, and being updated as the attributes are set, changed, or removed.

When a `canvas` element represents embedded content, it provides a paint source whose width is the element's intrinsic width, whose height is the element's intrinsic height, and whose appearance is the element's bitmap.

Whenever the `width` and `height` content attributes are set, removed, changed, or redundantly set to the value they already have, if the canvas context mode is direct-2d, the user agent must set bitmap dimensions to the numeric values of the `width` and `height` content attributes.

The `width` and `height` IDL attributes must reflect the respective content attributes of the same name, with the same defaults.

context = canvas . `getContext`(contextId [, ... ] )

  Returns an object that exposes an API for drawing on the canvas. The first argument specifies the desired API, either "`2d`" or "`webgl`". Subsequent arguments are handled by that API.

  This specification defines the "`2d`" context below. There is also a specification that defines a "`webgl`" context. [WEBGL]

  Returns null if the given context ID is not supported, if the canvas has already been initialised with the other context type (e.g. trying to get a "`2d`" context after getting a "`webgl`" context).

  Throws an `InvalidStateError` exception if the `setContext()` or `transferControlToProxy()` methods have been used.

supported = canvas . `probablySupportsContext`(contextId [, ... ] )

  Returns false if calling `getContext()` with the same arguments would definitely return null, and true otherwise.

  This return value is not a guarantee that `getContext()` will or will not return an object, as conditions (e.g. availability of system resources) can vary over time.

  Throws an `InvalidStateError` exception if the `setContext()` or `transferControlToProxy()` methods have been used.

canvas . `setContext`(context)

  Sets the `canvas`' rendering context to the given object.

  Throws an `InvalidStateError` exception if the `getContext()` or `transferControlToProxy()` methods have been used.

There are two ways for a `canvas` element to acquire a rendering context: the `canvas` element can provide one via the `getContext()` method, and one can be assigned to it via the `setContext()` method. In addition, the whole issue of a rendering context can be taken out of the `canvas` element's hands and passed to a `CanvasProxy` object, which itself can then be assigned a rendering context using its `setContext()` method.

These three methods are mutually exclusive; calling any of the three makes the other two start throwing `InvalidStateError` exceptions when called.

Each rendering context has a context bitmap mode, which is one of fixed, unbound, or bound. Initially, rendering contexts must be in the unbound mode.

The `getContext(contextId, arguments...)` method of the `canvas` element, when invoked, must run the steps in the cell of the following table whose column header describes the `canvas` element's canvas context mode and whose row header describes the method's first argument.

| none | direct-2d | direct-webgl | indirect | proxied
"`2d`" | Follow the 2D context creation algorithm defined in the section below, passing it the `canvas` element and the method's arguments..., to obtain a `CanvasRenderingContext2D` object; if this does not throw an exception, then set the `canvas` element's context mode to direct-2d, set the `CanvasRenderingContext2D` object's context bitmap mode to fixed, and return the `CanvasRenderingContext2D` object | Return the same object as was return the last time the method was invoked with this same first argument. | Return null. | Throw an `InvalidStateError` exception. | Throw an `InvalidStateError` exception.
"`webgl`", if the user agent supports the WebGL feature in its current configuration | Follow the instructions given in the WebGL specification's Context Creation section to obtain either a `WebGLRenderingContext` or null; if the returned value is null, then return null and abort these steps, otherwise, set the `canvas` element's context mode to direct-webgl, set the new `WebGLRenderingContext` object's context bitmap mode to fixed, and return the `WebGLRenderingContext` object [WEBGL] | Return null. | Return the same object as was return the last time the method was invoked with this same first argument. | Throw an `InvalidStateError` exception. | Throw an `InvalidStateError` exception.
A vendor-specific extension* | Behave as defined for the extension. | Behave as defined for the extension. | Behave as defined for the extension. | Throw an `InvalidStateError` exception. | Throw an `InvalidStateError` exception.
An unsupported value† | Return null. | Return null. | Return null. | Throw an `InvalidStateError` exception. | Throw an `InvalidStateError` exception.

* Vendors may define experimental contexts using the syntax `vendorname-context`, for example, `moz-3d`.

† For example, the "`webgl`" value in the case of a user agent having exhausted the graphics hardware's abilities and having no software fallback implementation.

The `probablySupportsContext(contextId, arguments...)` method of the `canvas` element, when invoked, must return false if calling `getContext()` on the same object and with the same arguments would definitely return null at this time, and true otherwise.

The `setContext(context)` method of the `canvas` element, when invoked, must run the following steps:

1. If the `canvas` element's canvas context mode is neither none nor indirect, throw an `InvalidStateError` exception and abort these steps.

2. If context's context bitmap mode is fixed, then throw an `InvalidStateError` exception and abort these steps.

3. If context's context bitmap mode is bound, then run context's unbinding steps and set its context's context bitmap mode to unbound.

4. Run context's binding steps to bind it to this `canvas` element.

5. Set the `canvas` element's context mode to indirect and the context's context bitmap mode to bound.

url = canvas . `toDataURL`( [ type, ... ] )

  Returns a `data:` URL for the image in the canvas.

  The first argument, if provided, controls the type of the image to be returned (e.g. PNG or JPEG). The default is `image/png`; that type is also used if the given type isn't supported. The other arguments are specific to the type, and control the way that the image is generated, as given in the table below.

  When trying to use types other than "`image/png`", authors can check if the image was really returned in the requested format by checking to see if the returned string starts with one of the exact strings "`data:image/png,`" or "`data:image/png;`". If it does, the image is PNG, and thus the requested type was not supported. (The one exception to this is if the canvas has either no height or no width, in which case the result might simply be "`data:,`".)

canvas . `toBlob`(callback [, type, ... ] )

  Creates a `Blob` object representing a file containing the image in the canvas, and invokes a callback with a handle to that object.

  The second argument, if provided, controls the type of the image to be returned (e.g. PNG or JPEG). The default is `image/png`; that type is also used if the given type isn't supported. The other arguments are specific to the type, and control the way that the image is generated, as given in the table below.

The `toDataURL()` method must run the following steps:

1. If the `canvas` element's bitmap's origin-clean flag is set to false, throw a `SecurityError` exception and abort these steps.

2. If the `canvas` element's bitmap has no pixels (i.e. either its horizontal dimension or its vertical dimension is zero) then return the string "`data:,`" and abort these steps. (This is the shortest `data:` URL; it represents the empty string in a `text/plain` resource.)

3. Let file be a serialisation of the `canvas` element's bitmap as a file, using the method's arguments (if any) as the arguments.

4. Return a `data:` URL representing file. [RFC2397]

The `toBlob()` method must run the following steps:

1. If the `canvas` element's bitmap's origin-clean flag is set to false, throw a `SecurityError` exception and abort these steps.

2. Let callback be the first argument.

3. Let arguments be the second and subsequent arguments to the method, if any.

4. If the `canvas` element's bitmap has no pixels (i.e. either its horizontal dimension or its vertical dimension is zero) then let result be null.

   Otherwise, let result be a `Blob` object representing a serialisation of the `canvas` element's bitmap as a file, using arguments. [FILEAPI]

5. Return, but continue running these steps in parallel.

6. If callback is null, abort these steps.

7. Queue a task to invoke the `FileCallback` callback with result as its argument. The task source for this task is the canvas blob serialisation task source.

##### 4.12.4.1 Proxying canvases to workers

Since DOM nodes cannot be accessed across worker boundaries, a proxy object is needed to enable workers to render to `canvas` elements in `Document`s.

    [Exposed=(Window,Worker)]
    interface CanvasProxy {
      void setContext(RenderingContext context);
    };
    // CanvasProxy implements Transferable;

canvasProxy = canvas . `transferControlToProxy`()

  Returns a `CanvasProxy` object that can be used to transfer control for this canvas over to another document (e.g. an `iframe` from another origin) or to a worker.

  Throws an `InvalidStateError` exception if the `getContext()` or `setContext()` methods have been used.

canvasProxy . `setContext`(context)

  Sets the `CanvasProxy` object's `canvas` element's rendering context to the given object.

  Throws an `InvalidStateError` exception if the `CanvasProxy` has been transfered.

The `transferControlToProxy()` method of the `canvas` element, when invoked, must run the following steps:

1. If the `canvas` element's canvas context mode is not none, throw an `InvalidStateError` exception and abort these steps.

2. Set the `canvas` element's context mode to proxied.

3. Return a `CanvasProxy` object bound to this `canvas` element.

A `CanvasProxy` object can be neutered (like any `Transferable` object), meaning it can no longer be transferred, and can be disabled, meaning it can no longer be bound to rendering contexts. When first created, a `CanvasProxy` object must be neither.

A `CanvasProxy` is created with a link to a `canvas` element. A `CanvasProxy` object that has not been disabled must have a strong reference to its canvas element.

The `setContext(context)` method of `CanvasProxy` objects, when invoked, must run the following steps:

1. If the `CanvasProxy` object has been disabled, throw an `InvalidStateError` exception and abort these steps.

2. If the `CanvasProxy` object has not been neutered, then neuter it.

3. If context's context bitmap mode is fixed, then throw an `InvalidStateError` exception and abort these steps.

4. If context's context bitmap mode is bound, then run context's unbinding steps and set its context's context bitmap mode to unbound.

5. Run context's binding steps to bind it to this `CanvasProxy` object's `canvas` element.

6. Set the context's context bitmap mode to bound.

To transfer a `CanvasProxy` object old to a new owner owner, a user agent must create a new `CanvasProxy` object linked to the same `canvas` element as old, thus obtaining new, must neuter and disable the old object, and must finally return new.

Here is a clock implemented on a worker. First, the main page:

    <!DOCTYPE HTML>
    <title>Clock</title>
    <canvas></canvas>
    <script>
      var canvas = document.getElementsByTagName('canvas')[0];
      var proxy = canvas.transferControlToProxy();
      var worker = new Worker('clock.js');
      worker.postMessage(proxy, [proxy]);
    </script>

Second, the worker:

    onmessage = function (event) {
      var context = new CanvasRenderingContext2D();
      event.data.setContext(context); // event.data is the CanvasProxy object
      setInterval(function () {
        context.clearRect(0, 0, context.width, context.height);
        context.fillText(new Date(), 0, 100);
        context.commit();
      }, 1000);
    };

##### 4.12.4.2 The 2D rendering context

    typedef (HTMLImageElement or
             HTMLVideoElement or
             HTMLCanvasElement or
             CanvasRenderingContext2D or
             ImageBitmap) CanvasImageSource;

    enum CanvasFillRule { "nonzero", "evenodd" };

    dictionary CanvasRenderingContext2DSettings {
      boolean alpha = true;
    };

    [Constructor(),
     Constructor(unsigned long width, unsigned long height),
     Exposed=(Window,Worker)]
    interface CanvasRenderingContext2D {

      // back-reference to the canvas
      readonly attribute HTMLCanvasElement canvas;

      // canvas dimensions
      attribute unsigned long width;
      attribute unsigned long height;

      // for contexts that aren't directly fixed to a specific canvas
      void commit(); // push the image to the output bitmap

      // state
      void save(); // push state on state stack
      void restore(); // pop state stack and restore state

      // transformations (default transform is the identity matrix)
      attribute SVGMatrix currentTransform;
      void scale(unrestricted double x, unrestricted double y);
      void rotate(unrestricted double angle);
      void translate(unrestricted double x, unrestricted double y);
      void transform(unrestricted double a, unrestricted double b, unrestricted double c, unrestricted double d, unrestricted double e, unrestricted double f);
      void setTransform(unrestricted double a, unrestricted double b, unrestricted double c, unrestricted double d, unrestricted double e, unrestricted double f);
      void resetTransform();

      // compositing
      attribute unrestricted double globalAlpha; // (default 1.0)
      attribute DOMString globalCompositeOperation; // (default source-over)

      // image smoothing
      attribute boolean imageSmoothingEnabled; // (default true)

      // colours and styles (see also the CanvasDrawingStyles interface)
      attribute (DOMString or CanvasGradient or CanvasPattern) strokeStyle; // (default black)
      attribute (DOMString or CanvasGradient or CanvasPattern) fillStyle; // (default black)
      CanvasGradient createLinearGradient(double x0, double y0, double x1, double y1);
      CanvasGradient createRadialGradient(double x0, double y0, double r0, double x1, double y1, double r1);
      CanvasPattern createPattern(CanvasImageSource image, [TreatNullAs=EmptyString] DOMString repetition);

      // shadows
      attribute unrestricted double shadowOffsetX; // (default 0)
      attribute unrestricted double shadowOffsetY; // (default 0)
      attribute unrestricted double shadowBlur; // (default 0)
      attribute DOMString shadowColor; // (default transparent black)

      // rects
      void clearRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
      void fillRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
      void strokeRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);

      // path API (see also CanvasPathMethods)
      void beginPath();
      void fill(optional CanvasFillRule fillRule = "nonzero");
      void fill(Path2D path, optional CanvasFillRule fillRule = "nonzero");
      void stroke();
      void stroke(Path2D path);
      void drawFocusIfNeeded(Element element);
      void drawFocusIfNeeded(Path2D path, Element element);
      void scrollPathIntoView();
      void scrollPathIntoView(Path2D path);
      void clip(optional CanvasFillRule fillRule = "nonzero");
      void clip(Path2D path, optional CanvasFillRule fillRule = "nonzero");
      void resetClip();
      boolean isPointInPath(unrestricted double x, unrestricted double y, optional CanvasFillRule fillRule = "nonzero");
      boolean isPointInPath(Path2D path, unrestricted double x, unrestricted double y, optional CanvasFillRule fillRule = "nonzero");
      boolean isPointInStroke(unrestricted double x, unrestricted double y);
      boolean isPointInStroke(Path2D path, unrestricted double x, unrestricted double y);

      // text (see also the CanvasDrawingStyles interface)
      void fillText(DOMString text, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth);
      void strokeText(DOMString text, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth);
      TextMetrics measureText(DOMString text);

      // drawing images
      void drawImage(CanvasImageSource image, unrestricted double dx, unrestricted double dy);
      void drawImage(CanvasImageSource image, unrestricted double dx, unrestricted double dy, unrestricted double dw, unrestricted double dh);
      void drawImage(CanvasImageSource image, unrestricted double sx, unrestricted double sy, unrestricted double sw, unrestricted double sh, unrestricted double dx, unrestricted double dy, unrestricted double dw, unrestricted double dh);

      // hit regions
      void addHitRegion(optional HitRegionOptions options);
      void removeHitRegion(DOMString id);
      void clearHitRegions();

      // pixel manipulation
      ImageData createImageData(double sw, double sh);
      ImageData createImageData(ImageData imagedata);
      ImageData getImageData(double sx, double sy, double sw, double sh);
      void putImageData(ImageData imagedata, double dx, double dy);
      void putImageData(ImageData imagedata, double dx, double dy, double dirtyX, double dirtyY, double dirtyWidth, double dirtyHeight);
    };
    CanvasRenderingContext2D implements CanvasDrawingStyles;
    CanvasRenderingContext2D implements CanvasPathMethods;

    [NoInterfaceObject, Exposed=(Window,Worker)]
    interface CanvasDrawingStyles {
      // line caps/joins
      attribute unrestricted double lineWidth; // (default 1)
      attribute DOMString lineCap; // "butt", "round", "square" (default "butt")
      attribute DOMString lineJoin; // "round", "bevel", "miter" (default "miter")
      attribute unrestricted double miterLimit; // (default 10)

      // dashed lines
      void setLineDash(sequence<unrestricted double> segments); // default empty
      sequence<unrestricted double> getLineDash();
      attribute unrestricted double lineDashOffset;

      // text
      attribute DOMString font; // (default 10px sans-serif)
      attribute DOMString textAlign; // "start", "end", "left", "right", "center" (default: "start")
      attribute DOMString textBaseline; // "top", "hanging", "middle", "alphabetic", "ideographic", "bottom" (default: "alphabetic")
      attribute DOMString direction; // "ltr", "rtl", "inherit" (default: "inherit")
    };

    [NoInterfaceObject, Exposed=(Window,Worker)]
    interface CanvasPathMethods {
      // shared path API methods
      void closePath();
      void moveTo(unrestricted double x, unrestricted double y);
      void lineTo(unrestricted double x, unrestricted double y);
      void quadraticCurveTo(unrestricted double cpx, unrestricted double cpy, unrestricted double x, unrestricted double y);
      void bezierCurveTo(unrestricted double cp1x, unrestricted double cp1y, unrestricted double cp2x, unrestricted double cp2y, unrestricted double x, unrestricted double y);
      void arcTo(unrestricted double x1, unrestricted double y1, unrestricted double x2, unrestricted double y2, unrestricted double radius);
      void arcTo(unrestricted double x1, unrestricted double y1, unrestricted double x2, unrestricted double y2, unrestricted double radiusX, unrestricted double radiusY, unrestricted double rotation);
      void rect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
      void arc(unrestricted double x, unrestricted double y, unrestricted double radius, unrestricted double startAngle, unrestricted double endAngle, optional boolean anticlockwise = false);
      void ellipse(unrestricted double x, unrestricted double y, unrestricted double radiusX, unrestricted double radiusY, unrestricted double rotation, unrestricted double startAngle, unrestricted double endAngle, optional boolean anticlockwise = false);
    };

    [Exposed=(Window,Worker)]
    interface CanvasGradient {
      // opaque object
      void addColorStop(double offset, DOMString color);
    };

    [Exposed=(Window,Worker)]
    interface CanvasPattern {
      // opaque object
      void setTransform(SVGMatrix transform);
    };

    [Exposed=(Window,Worker)]
    interface TextMetrics {
      // x-direction
      readonly attribute double width; // advance width
      readonly attribute double actualBoundingBoxLeft;
      readonly attribute double actualBoundingBoxRight;

      // y-direction
      readonly attribute double fontBoundingBoxAscent;
      readonly attribute double fontBoundingBoxDescent;
      readonly attribute double actualBoundingBoxAscent;
      readonly attribute double actualBoundingBoxDescent;
      readonly attribute double emHeightAscent;
      readonly attribute double emHeightDescent;
      readonly attribute double hangingBaseline;
      readonly attribute double alphabeticBaseline;
      readonly attribute double ideographicBaseline;
    };

    dictionary HitRegionOptions {
      Path2D? path = null;
      CanvasFillRule fillRule = "nonzero";
      DOMString id = "";
      DOMString? parentID = null;
      DOMString cursor = "inherit";
      // for control-backed regions:
      Element? control = null;
      // for unbacked regions:
      DOMString? label = null;
      DOMString? role = null;
    };

    [Constructor(unsigned long sw, unsigned long sh),
     Constructor(Uint8ClampedArray data, unsigned long sw, optional unsigned long sh),
     Exposed=(Window,Worker)]
    interface ImageData {
      readonly attribute unsigned long width;
      readonly attribute unsigned long height;
      readonly attribute Uint8ClampedArray data;
    };

    [Constructor(optional Element scope), Exposed=(Window,Worker)]
    interface DrawingStyle { };
    DrawingStyle implements CanvasDrawingStyles;

    [Constructor,
     Constructor(Path2D path),
     Constructor(Path2D[] paths, optional CanvasFillRule fillRule = "nonzero"),
     Constructor(DOMString d), Exposed=(Window,Worker)]
    interface Path2D {
      void addPath(Path2D path, optional SVGMatrix? transformation = null);
      void addPathByStrokingPath(Path2D path, CanvasDrawingStyles styles, optional SVGMatrix? transformation = null);
      void addText(DOMString text, CanvasDrawingStyles styles, SVGMatrix? transformation, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth);
      void addPathByStrokingText(DOMString text, CanvasDrawingStyles styles, SVGMatrix? transformation, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth);
      void addText(DOMString text, CanvasDrawingStyles styles, SVGMatrix? transformation, Path2D path, optional unrestricted double maxWidth);
      void addPathByStrokingText(DOMString text, CanvasDrawingStyles styles, SVGMatrix? transformation, Path2D path, optional unrestricted double maxWidth);
    };
    Path2D implements CanvasPathMethods;

context = canvas . `getContext`('2d' [, { [ `alpha`: false ] } ] )

  Returns a `CanvasRenderingContext2D` object that is permanently bound to a particular `canvas` element.

  If the `alpha` setting is provided and set to false, then the canvas is forced to always be opaque.

context = new `CanvasRenderingContext2D`( [ width, height ] )

  Returns an unbound `CanvasRenderingContext2D` object with an implied bitmap with the given dimensions in CSS pixels (300x150, if the arguments are omitted).

context . `canvas`

  Returns the `canvas` element, if the rendering context was obtained using the `getContext()` method.

context . `width`
context . `height`

  Return the dimensions of the bitmap, in CSS pixels.

  Can be set, to update the bitmap's dimensions. If the rendering context is bound to a canvas, this will also update the canvas' intrinsic dimensions.

context . `commit`()

  If the rendering context is bound to a `canvas`, display the current frame.

A `CanvasRenderingContext2D` object can be obtained in two ways: the `getContext()` method on a `canvas` element (which invokes the 2D context creation algorithm), and the `CanvasRenderingContext2D()` constructor.

A `CanvasRenderingContext2D` object has a scratch bitmap and can be bound to an output bitmap. These are initialised when the object is created, and can be subsequently adjusted when the rendering context is bound or unbound. In some cases, these bitmaps are the same underlying bitmap. In general, the scratch bitmap is what scripts interact with, and the output bitmap is what is being displayed. These bitmaps always have the same dimensions.

Each such bitmap has an origin-clean flag, which can be set to true or false. Initially, when one of these bitmaps is created, its origin-clean flag must be set to true.

These bitmaps also have a hit region list, which is described in a later section. Initially, this list is empty.

Scratch bitmaps have a list of pending interface actions, which can contain instructions to draw the user's attention to a location on the bitmap, and instructions to scroll to a location on the bitmap. Initially, this list is also empty.

Scratch bitmaps also have an alpha flag, which can be set to true or false. Initially, when one of these bitmaps is created, its alpha flag must be set to true. When a scratch bitmap has its alpha flag set to false, then its alpha channel must be fixed to 1.0 (fully opaque) for all pixels, and attempts to change the alpha component of any pixel must be silently ignored.

Thus, such a bitmap starts off as fully-opaque black instead of fully-tranparent black; `clearRect()` always results in fully-opaque black pixels, every fourth byte from `getImageData()` is always 255, the `putImageData()` method effectively ignores every fourth byte in its input, and so on. However, the alpha component of styles and images drawn onto the canvas are still honoured up to the point where they would impact the scratch bitmap's alpha channel; for instance, drawing a 50% transparent white square on a freshly created scratch bitmap with its alpha flag set to false will result in a fully-opaque gray square.

The `CanvasRenderingContext2D` 2D rendering context represents a flat linear Cartesian surface whose origin (0,0) is at the top left corner, with the coordinate space having x values increasing when going right, and y values increasing when going down. The x-coordinate of the right-most edge is equal to the width of the rendering context's scratch bitmap in CSS pixels; similarly, the y-coordinate of the bottom-most edge is equal to the height of the rendering context's scratch bitmap in CSS pixels.

The size of the coordinate space does not necessarily represent the size of the actual bitmaps that the user agent will use internally or during rendering. On high-definition displays, for instance, the user agent may internally use bitmaps with two device pixels per unit in the coordinate space, so that the rendering remains at high quality throughout. Anti-aliasing can similarly be implemented using over-sampling with bitmaps of a higher resolution than the final image on the display.

The 2D context creation algorithm, which is passed a target (a `canvas` element) and optionally some arguments, consists of running the following steps:

1. If the algorithm was passed some arguments, let arg be the first such argument. Otherwise, let arg be undefined.

2. Let settings be the result of coercing the arg context arguments for 2D.

3. Create a new `CanvasRenderingContext2D` object.

4. Initialise its `canvas` attribute to point to target.

5. Let the new `CanvasRenderingContext2D` object's output bitmap and scratch bitmap both be the same bitmap as target's bitmap (so that they are shared).

6. Set bitmap dimensions to the numeric values of target's `width` and `height` content attributes.

7. Process each of the members of settings as follows:

   `alpha`
     If false, then set the target's bitmap's alpha flag to false.

8. Return the new `CanvasRenderingContext2D` object.

The `CanvasRenderingContext2D()` constructor, when invoked, must run the following steps:

1. Create a new `CanvasRenderingContext2D` object.

2. Initialise its `canvas` attribute to null.

3. Let the new `CanvasRenderingContext2D` object's scratch bitmap be a new bitmap.

4. If the constructor was called with arguments, let width and height be the first and second arguments, respectively. Otherwise, let width and height be 300 and 150, respectively.

5. Set bitmap dimensions to width and height.

6. Let the new `CanvasRenderingContext2D` object have no output bitmap.

7. Return the new `CanvasRenderingContext2D` object.

When a user agent is required to coerce context arguments for 2D, it must run the following steps:

1. Let input be the argument to coerce.

2. Let jsval be the result of converting input to an ECMAScript value. If this throws an exception, then propagate the exception and abort these steps.

3. Let dict be the result of converting jsval to the dictionary type `CanvasRenderingContext2DSettings`. If this throws an exception, then propagate the exception and abort these steps.

4. Return dict.

When the user agent is required to commit the scratch bitmap for a rendering context, it must run the following steps:

1. Let bitmap copy be a copy of the rendering context's scratch bitmap.

2. Let origin-clean flag copy be a copy of the rendering context's scratch bitmap's origin-clean flag.

3. Let hit region list copy be a copy of the rendering context's scratch bitmap's hit region list.

4. Let list of pending interface actions copy be a copy of the rendering context's scratch bitmap's list of pending interface actions.

5. Empty the scratch bitmap's list of pending interface actions.

6. If the rendering context has no output bitmap, abort these steps.

7. Let output bitmap be the rendering context's output bitmap.

8. Let canvas be the `canvas` element to which the rendering context was most recently bound.

9. Queue a task associated with canvas' `Document` to perform the following substeps:

   1. Overwrite output bitmap with bitmap copy.

   2. Overwrite output bitmap's origin-clean flag with origin-clean flag copy.

   3. Overwrite output bitmap's hit region list with hit region list copy.

   4. Follow the directions in the list of pending interface actions copy.

The algorithm above must use the canvas updating task source (which is just used by this algorithm).

The `commit()` method must run the following steps:

1. If the rendering context's context bitmap mode is fixed, throw an `InvalidStateError` exception and abort these steps.

2. Commit the scratch bitmap for the rendering context.

The scratch bitmap is only committed when the `commit()` method is called. (This doesn't matter for `canvas` elements in direct-2d mode, since there the scratch bitmap is also the `canvas` element's bitmap so every drawing operation is immediately drawn.)

When the user agent is to set bitmap dimensions to width and height, it must run the following steps:

1. Reset the rendering context to its default state.

2. Clear the scratch bitmap's hit region list and its list of pending interface actions.

3. Resize the scratch bitmap to the new width and height and clear it to fully transparent black.

4. If the rendering context has an output bitmap, and the scratch bitmap is a different bitmap than the output bitmap, then resize the output bitmap to the new width and height and clear it to fully transparent black.

5. If the rendering context's context bitmap mode is fixed, then run these substeps:

   1. Let canvas be the `canvas` element to which the rendering context's `canvas` attribute was initialised.

   2. If the rendering context's context bitmap mode is fixed and the numeric value of the canvas' `width` content attribute differs from width, then set canvas' `width` content attribute to the shortest possible string representing width as a valid non-negative integer.

   3. If the rendering context's context bitmap mode is fixed and the numeric value of the canvas' `height` content attribute differs from height, then set canvas' `height` content attribute to the shortest possible string representing height as a valid non-negative integer.

Only one square appears to be drawn in the following example:

    // canvas is a reference to a <canvas> element
    var context = canvas.getContext('2d');
    context.fillRect(0,0,50,50);
    canvas.setAttribute('width', '300'); // clears the canvas
    context.fillRect(0,100,50,50);
    canvas.width = canvas.width; // clears the canvas
    context.fillRect(100,0,50,50); // only this square remains

When the user agent is to run the unbinding steps for a rendering context, it must run the following steps:

1. Reset the rendering context to its default state.

2. Clear the scratch bitmap's hit region list and its list of pending interface actions.

3. Clear the `CanvasRenderingContext2D` object's scratch bitmap to a transparent black.

4. Set the `CanvasRenderingContext2D` object's scratch bitmap's origin-clean flag to true.

5. Let the `CanvasRenderingContext2D` object have no output bitmap.

When the user agent is to run the binding steps to bind the rendering context to the `canvas` element target, it must run the following steps:

1. Reset the rendering context to its default state.

2. Clear the scratch bitmap's hit region list and its list of pending interface actions.

3. Resize the `CanvasRenderingContext2D` object's scratch bitmap to the dimensions of target's bitmap and clear it to fully transparent black.

4. Set the `CanvasRenderingContext2D` object's scratch bitmap's origin-clean flag to true.

5. Let the `CanvasRenderingContext2D` object's output bitmap be target's bitmap.

The `canvas` attribute must return the value it was initialised to when the object was created.

The `width` attribute, on getting, must return the width of the rendering context's scratch bitmap, in CSS pixels. On setting, it must set bitmap dimensions to the new value and the current height of the rendering context's scratch bitmap in CSS pixels, respectively.

The `height` attribute, on getting, must return the height of the rendering context's scratch bitmap, in CSS pixels. On setting, it must set bitmap dimensions to the current width of the rendering context's scratch bitmap in CSS pixels and the new value, respectively.

Except where otherwise specified, for the 2D context interface, any method call with a numeric argument whose value is infinite or a NaN value must be ignored.

Whenever the CSS value `currentColor` is used as a colour in the `CanvasRenderingContext2D` API, the "computed value of the 'color' property" for the purposes of determining the computed value of the `currentColor` keyword is the value described by the appropriate entry in the following list:

If the rendering context's context bitmap mode is fixed and the `canvas` element is being rendered

  The "computed value of the 'color' property" for the purposes of determining the computed value of the `currentColor` keyword is the computed value of the 'color' property on the `canvas` element at the time that the colour is specified (e.g. when the appropriate attribute is set, or when the method is called; not when the colour is rendered or otherwise used). [CSSCOLOR]

In all other cases

  The "computed value of the 'color' property" for the purposes of determining the computed value of the `currentColor` keyword is fully opaque black. [CSSCOLOR]

In the case of `addColorStop()` on `CanvasGradient`, the "computed value of the 'color' property" for the purposes of determining the computed value of the `currentColor` keyword is always fully opaque black (there is no associated element). [CSSCOLOR]

This is because `CanvasGradient` objects are `canvas`-neutral — a `CanvasGradient` object created by one `canvas` can be used by another, and there is therefore no way to know which is the "element in question" at the time that the colour is specified.

Similar concerns exist with font-related properties; the rules for those are described in detail in the relevant section below.

The `CanvasFillRule` enumeration is used to select the fill rule algorithm by which to determine if a point is inside or outside a path.

The value "`nonzero`" value indicates the non-zero winding rule, wherein a point is considered to be outside a shape if the number of times a half-infinite straight line drawn from that point crosses the shape's path going in one direction is equal to the number of times it crosses the path going in the other direction.

The "`evenodd`" value indicates the even-odd rule, wherein a point is considered to be outside a shape if the number of times a half-infinite straight line drawn from that point crosses the shape's path is even.

If a point is not outside a shape, it is inside the shape.

###### 4.12.4.2.1 Implementation notes

This section is non-normative.

Although the way the specification is written it might sound like an implementation needs to track up to four bitmaps per canvas or rendering context — one scratch bitmap, one output bitmap for the rendering context, one bitmap for the `canvas`, and one bitmap for the actually currently rendered image — user agents can in fact generally optimise this to only one or two.

The scratch bitmap, when it isn't the same bitmap as the output bitmap, is only directly observable if it is read, and therefore implementations can, instead of updating this bitmap, merely remember the sequence of drawing operations that have been applied to it until such time as the bitmap's actual data is needed (for example because of a call to `commit()`, `drawImage()`, or the `createImageBitmap()` factory method). In many cases, this will be more memory efficient.

The bitmap of a `canvas` element is the one bitmap that's pretty much always going to be needed in practice. The output bitmap of a rendering context, when it has one, is always just an alias to a `canvas` element's bitmap.

Additional bitmaps are sometimes needed, e.g. to enable fast drawing when the canvas is being painted at a different size than its intrinsic size, or to enable double buffering so that the rendering commands from the scratch bitmap can be applied without the rendering being updated midway.

###### 4.12.4.2.2 The canvas state

Each `CanvasRenderingContext2D` rendering context maintains a stack of drawing states. Drawing states consist of:

- The current transformation matrix.
- The current clipping region.
- The current values of the following attributes: `strokeStyle`, `fillStyle`, `globalAlpha`, `lineWidth`, `lineCap`, `lineJoin`, `miterLimit`, `lineDashOffset`, `shadowOffsetX`, `shadowOffsetY`, `shadowBlur`, `shadowColor`, `globalCompositeOperation`, `font`, `textAlign`, `textBaseline`, `direction`, `imageSmoothingEnabled`.
- The current dash list.

The current default path and the rendering context's bitmaps are not part of the drawing state. The current default path is persistent, and can only be reset using the `beginPath()` method. The bitmaps depend on whether and how the rendering context is bound to a `canvas` element.

context . `save`()

  Pushes the current state onto the stack.

context . `restore`()

  Pops the top state on the stack, restoring the context to that state.

The `save()` method must push a copy of the current drawing state onto the drawing state stack.

The `restore()` method must pop the top entry in the drawing state stack, and reset the drawing state it describes. If there is no saved state, the method must do nothing.

When the user agent is to reset the rendering context to its default state, it must clear the drawing state stack and everything that drawing state consists of to initial values.

###### 4.12.4.2.3 `DrawingStyle` objects

All the line styles (line width, caps, joins, and dash patterns) and text styles (fonts) described in the next two sections apply to `CanvasRenderingContext2D` objects and to `DrawingStyle` objects. This section defines the constructor used to obtain a `DrawingStyle` object. This object is then used by methods on `Path2D` objects to control how text and paths are rasterised and stroked.

styles = new `DrawingStyle`( [ element ] )

  Creates a new `DrawingStyle` object, optionally using a specific element for resolving relative keywords and sizes in font specifications.

Each `DrawingStyle` object can have a styles scope object.

The `DrawingStyle()` constructor, when invoked, must return a newly created `DrawingStyle` object. If the constructor was passed an argument, then the `DrawingStyle` object's styles scope object is that element. Otherwise, if the JavaScript global environment is a document environment, the object's styles scope object is the `Document` object of the active document of the browsing context of the `Window` object on which the interface object of the invoked constructor is found. Otherwise, the JavaScript global environment is a worker environment, and the styles scope object is the worker.

###### 4.12.4.2.4 Line styles

context . `lineWidth` [ = value ]
styles . `lineWidth` [ = value ]

  Returns the current line width.

  Can be set, to change the line width. Values that are not finite values greater than zero are ignored.

context . `lineCap` [ = value ]
styles . `lineCap` [ = value ]

  Returns the current line cap style.

  Can be set, to change the line cap style.

  The possible line cap styles are "`butt"`", "`round"`", and "`square"`". Other values are ignored.

context . `lineJoin` [ = value ]
styles . `lineJoin` [ = value ]

  Returns the current line join style.

  Can be set, to change the line join style.

  The possible line join styles are "`bevel"`", "`round"`", and "`miter"`". Other values are ignored.

context . `miterLimit` [ = value ]
styles . `miterLimit` [ = value ]

  Returns the current miter limit ratio.

  Can be set, to change the miter limit ratio. Values that are not finite values greater than zero are ignored.

context . `setLineDash`(segments)
styles . `setLineDash`(segments)

  Sets the current line dash pattern (as used when stroking). The argument is a list of distances for which to alternately have the line on and the line off.

segments = context . `getLineDash`()
segments = styles . `getLineDash`()

  Returns a copy of the current line dash pattern. The array returned will always have an even number of entries (i.e. the pattern is normalized).

context . `lineDashOffset`
styles . `lineDashOffset`

  Returns the phase offset (in the same units as the line dash pattern).

  Can be set, to change the phase offset. Values that are not finite values are ignored.

Objects that implement the `CanvasDrawingStyles` interface have attributes and methods (defined in this section) that control how lines are treated by the object.

The `lineWidth` attribute gives the width of lines, in coordinate space units. On getting, it must return the current value. On setting, zero, negative, infinite, and NaN values must be ignored, leaving the value unchanged; other values must change the current value to the new value.

When the object implementing the `CanvasDrawingStyles` interface is created, the `lineWidth` attribute must initially have the value 1.0.

The `lineCap` attribute defines the type of endings that UAs will place on the end of lines. The three valid values are "`butt"`", "`round"`", and "`square"`".

On getting, it must return the current value. On setting, if the new value is one of the literal strings "`butt"`", "`round"`", and "`square"`", then the current value must be changed to the new value; other values must ignored, leaving the value unchanged.

When the object implementing the `CanvasDrawingStyles` interface is created, the `lineCap` attribute must initially have the value "`butt"`".

The `lineJoin` attribute defines the type of corners that UAs will place where two lines meet. The three valid values are "`bevel"`", "`round"`", and "`miter"`".

On getting, it must return the current value. On setting, if the new value is one of the literal strings "`bevel"`", "`round"`", and "`miter"`", then the current value must be changed to the new value; other values must be ignored, leaving the value unchanged.

When the object implementing the `CanvasDrawingStyles` interface is created, the `lineJoin` attribute must initially have the value "`miter"`".

When the `lineJoin` attribute has the value "`miter"`", strokes use the miter limit ratio to decide how to render joins. The miter limit ratio can be explicitly set using the `miterLimit` attribute. On getting, it must return the current value. On setting, zero, negative, infinite, and NaN values must be ignored, leaving the value unchanged; other values must change the current value to the new value.

When the object implementing the `CanvasDrawingStyles` interface is created, the `miterLimit` attribute must initially have the value 10.0.

Each `CanvasDrawingStyles` object has a dash list, which is either empty or consists of an even number of non-negative numbers. Initially, the dash list must be empty.

When the `setLineDash()` method is invoked, it must run the following steps:

1. Let a be the argument.

2. If any value in a is not finite (e.g. an Infinity or a NaN value), or if any value is negative (less than zero), then abort these steps (without throwing an exception; user agents could show a message on a developer console, though, as that would be helpful for debugging).

3. If the number of elements in a is odd, then let a be the concatentation of two copies of a.

4. Let the object's dash list be a.

When the `getLineDash()` method is invoked, it must return a sequence whose values are the values of the object's dash list, in the same order.

It is sometimes useful to change the "phase" of the dash pattern, e.g. to achieve a "marching ants" effect. The phase can be set using the `lineDashOffset` attribute. On getting, it must return the current value. On setting, infinite and NaN values must be ignored, leaving the value unchanged; other values must change the current value to the new value.

When the object implementing the `CanvasDrawingStyles` interface is created, the `lineDashOffset` attribute must initially have the value 0.0.

When a user agent is to trace a path, given an object style that implements the `CanvasDrawingStyles` interface, it must run the following algorithm. This algorithm returns a new path.

1. Let path be a copy of the path being traced.

2. Prune all zero-length line segments from path.

3. Remove from path any subpaths containing no lines (i.e. subpaths with just one point).

4. Replace each point in each subpath of path other than the first point and the last point of each subpath by a join that joins the line leading to that point to the line leading out of that point, such that the subpaths all consist of two points (a starting point with a line leading out of it, and an ending point with a line leading into it), one or more lines (connecting the points and the joins), and zero or more joins (each connecting one line to another), connected together such that each subpath is a series of one or more lines with a join between each one and a point on each end.

5. Add a straight closing line to each closed subpath in path connecting the last point and the first point of that subpath; change the last point to a join (from the previously last line to the newly added closing line), and change the first point to a join (from the newly added closing line to the first line).

6. If the styles dash list is empty, jump to the step labeled convert.

7. Let pattern width be the concatenation of all the entries of the styles dash list, in coordinate space units.

8. For each subpath subpath in path, run the following substeps. These substeps mutate the subpaths in path in vivo.

   1. Let subpath width be the length of all the lines of subpath, in coordinate space units.

   2. Let offset be the value of the styles `lineDashOffset`, in coordinate space units.

   3. While offset is greater than pattern width, decrement it by pattern width.

      While offset is less than zero, increment it by pattern width.

   4. Define L to be a linear coordinate line defined along all lines in subpath, such that the start of the first line in the subpath is defined as coordinate 0, and the end of the last line in the subpath is defined as coordinate subpath width.

   5. Let position be zero minus offset.

   6. Let index be 0.

   7. Let current state be off (the other states being on and zero-on).

   8. Dash on: Let segment length be the value of the styles dash list's indexth entry.

   9. Increment position by segment length.

   10. If position is greater than subpath width, then end these substeps for this subpath and start them again for the next subpath; if there are no more subpaths, then jump to the step labeled convert instead.

   11. If segment length is non-zero, let current state be on.

   12. Increment index by one.

   13. Dash off: Let segment length be the value of the styles dash list's indexth entry.

   14. Let start be the offset position on L.

   15. Increment position by segment length.

   16. If position is less than zero, then jump to the step labeled post-cut.

   17. If start is less than zero, then let start be zero.

   18. If position is greater than subpath width, then let end be the offset subpath width on L. Otherwise, let end be the offset position on L.

   19. Jump to the first appropriate step:

       If segment length is zero and current state is off

         Do nothing, just continue to the next step.

       If current state is off

         Cut the line on which end finds itself short at end and place a point there, cutting the subpath that it was in in two; remove all line segments, joins, points, and subpaths that are between start and end; and finally place a single point at start with no lines connecting to it.

         The point has a directionality for the purposes of drawing line caps (see below). The directionality is the direction that the original line had at that point (i.e. when L was defined above).

       Otherwise

         Cut the line on which start finds itself into two at start and place a point there, cutting the subpath that it was in in two, and similarly cut the line on which end finds itself short at end and place a point there, cutting the subpath that it was in in two, and then remove all line segments, joins, points, and subpaths that are between start and end.

         If start and end are the same point, then this results in just the line being cut in two and two points being inserted there, with nothing being removed, unless a join also happens to be at that point, in which case the join must be removed.

   20. Post-cut: If position is greater than subpath width, then jump to the step labeled convert.

   21. If segment length is greater than zero, let positioned-at-on-dash be false.

   22. Increment index by one. If it is equal to the number of entries in the styles dash list, then let index be 0.

   23. Return to the step labeled dash on.

9. Convert: This is the step that converts the path to a new path that represents its stroke.

   Create a new path that describes the edge of the areas that would be covered if a straight line of length equal to the styles `lineWidth` was swept along each subpath in path while being kept at an angle such that the line is orthogonal to the path being swept, replacing each point with the end cap necessary to satisfy the styles `lineCap` attribute as described previously and elaborated below, and replacing each join with the join necessary to satisfy the styles `lineJoin` type, as defined below.

   Caps: Each point has a flat edge perpendicular to the direction of the line coming out of it. This is them augmented according to the value of the styles `lineCap`. The "`butt"`" value means that no additional line cap is added. The "`round"`" value means that a semi-circle with the diameter equal to the styles `lineWidth` width must additionally be placed on to the line coming out of each point. The "`square"`" value means that a rectangle with the length of the styles `lineWidth` width and the width of half the styles `lineWidth` width, placed flat against the edge perpendicular to the direction of the line coming out of the point, must be added at each point.

   Points with no lines coming out of them must have two caps placed back-to-back as if it was really two points connected to each other by an infinitesimally short straight line in the direction of the point's directionality (as defined above).

   Joins: In addition to the point where a join occurs, two additional points are relevant to each join, one for each line: the two corners found half the line width away from the join point, one perpendicular to each line, each on the side furthest from the other line.

   A triangle connecting these two opposite corners with a straight line, with the third point of the triangle being the join point, must be added at all joins. The `lineJoin` attribute controls whether anything else is rendered. The three aforementioned values have the following meanings:

   The "`bevel"`" value means that this is all that is rendered at joins.

   The "`round"`" value means that an arc connecting the two aforementioned corners of the join, abutting (and not overlapping) the aforementioned triangle, with the diameter equal to the line width and the origin at the point of the join, must be added at joins.

   The "`miter"`" value means that a second triangle must (if it can given the miter length) be added at the join, with one line being the line between the two aforementioned corners, abutting the first triangle, and the other two being continuations of the outside edges of the two joining lines, as long as required to intersect without going over the miter length.

   The miter length is the distance from the point where the join occurs to the intersection of the line edges on the outside of the join. The miter limit ratio is the maximum allowed ratio of the miter length to half the line width. If the miter length would cause the miter limit ratio (as set by the style `miterLimit` attribute) to be exceeded, this second triangle must not be added.

   The subpaths in the newly created path must be oriented such that for any point, the number of times a half-infinite straight line drawn from that point crosses a subpath is even if and only if the number of times a half-infinite straight line drawn from that same point crosses a subpath going in one direction is equal to the number of times it crosses a subpath going in the other direction.

10. Return the newly created path.

###### 4.12.4.2.5 Text styles

context . `font` [ = value ]
styles . `font` [ = value ]

  Returns the current font settings.

  Can be set, to change the font. The syntax is the same as for the CSS 'font' property; values that cannot be parsed as CSS font values are ignored.

  Relative keywords and lengths are computed relative to the font of the `canvas` element.

context . `textAlign` [ = value ]
styles . `textAlign` [ = value ]

  Returns the current text alignment settings.

  Can be set, to change the alignment. The possible values are and their meanings are given below. Other values are ignored. The default is "`start`".

context . `textBaseline` [ = value ]
styles . `textBaseline` [ = value ]

  Returns the current baseline alignment settings.

  Can be set, to change the baseline alignment. The possible values and their meanings are given below. Other values are ignored. The default is "`alphabetic`".

context . `direction` [ = value ]
styles . `direction` [ = value ]

  Returns the current directionality.

  Can be set, to change the directionality. The possible values and their meanings are given below. Other values are ignored. The default is "`inherit`".

Objects that implement the `CanvasDrawingStyles` interface have attributes (defined in this section) that control how text is laid out (rasterised or outlined) by the object. Such objects can also have a font style source object. For `CanvasRenderingContext2D` objects whose context bitmap mode is fixed, this is their `canvas` element; for other `CanvasRenderingContext2D` objects, if the JavaScript global environment is a document environment, the object's font style source object is the `Document` object of the active document of the browsing context of the `Window` object on which the interface object of the `CanvasRenderingContext2D` object is found; otherwise the JavaScript global environment is a worker environment and the font style source object is the worker. For `DrawingStyle` objects, it's the styles scope object.

The `font` IDL attribute, on setting, must be parsed as a CSS <'font'> value (but without supporting property-independent style sheet syntax like 'inherit'), and the resulting font must be assigned to the context, with the 'line-height' component forced to 'normal', with the 'font-size' component converted to CSS pixels, and with system fonts being computed to explicit values. If the new value is syntactically incorrect (including using property-independent style sheet syntax like 'inherit' or 'initial'), then it must be ignored, without assigning a new font value. [CSS]

Font family names must be interpreted in the context of the font style source object when the font is to be used; any fonts embedded using `@font-face` or loaded using `FontFace` objects that are visible to the font style source object must therefore be available once they are loaded. (Each font style source object has a font source, which determines what fonts are available.) If a font is used before it is fully loaded, or if the font style source object does not have that font in scope at the time the font is to be used, then it must be treated as if it was an unknown font, falling back to another as described by the relevant CSS specifications. [CSSFONTS] [CSSFONTLOAD]

On getting, the `font` attribute must return the serialised form of the current font of the context (with no 'line-height' component). [CSSOM]

For example, after the following statement:

    context.font = 'italic 400 12px/2 Unknown Font, sans-serif';

...the expression `context.font` would evaluate to the string "`italic 12px "Unknown Font", sans-serif`". The "400" font-weight doesn't appear because that is the default value. The line-height doesn't appear because it is forced to "normal", the default value.

When the object implementing the `CanvasDrawingStyles` interface is created, the font of the context must be set to 10px sans-serif. When the 'font-size' component is set to lengths using percentages, 'em' or 'ex' units, or the 'larger' or 'smaller' keywords, these must be interpreted relative to the computed value of the 'font-size' property of the font style source object at the time that the attribute is set, if it is an element. When the 'font-weight' component is set to the relative values 'bolder' and 'lighter', these must be interpreted relative to the computed value of the 'font-weight' property of the font style source object at the time that the attribute is set, if it is an element. If the computed values are undefined for a particular case (e.g. because the font style source object is not an element or is not being rendered), then the relative keywords must be interpreted relative to the normal-weight 10px sans-serif default.

The `textAlign` IDL attribute, on getting, must return the current value. On setting, if the value is one of `start`, `end`, `left`, `right`, or `center`, then the value must be changed to the new value. Otherwise, the new value must be ignored. When the object implementing the `CanvasDrawingStyles` interface is created, the `textAlign` attribute must initially have the value `start`.

The `textBaseline` IDL attribute, on getting, must return the current value. On setting, if the value is one of `top`, `hanging`, `middle`, `alphabetic`, `ideographic`, or `bottom`, then the value must be changed to the new value. Otherwise, the new value must be ignored. When the object implementing the `CanvasDrawingStyles` interface is created, the `textBaseline` attribute must initially have the value `alphabetic`.

The `direction` IDL attribute, on getting, must return the current value. On setting, if the value is one of "`ltr`", "`rtl`", or "`inherit`", then the value must be changed to the new value. Otherwise, the new value must be ignored. When the object implementing the `CanvasDrawingStyles` interface is created, the `direction` attribute must initially have the value "`inherit`".

The `textAlign` attribute's allowed keywords are as follows:

`start`

  Align to the start edge of the text (left side in left-to-right text, right side in right-to-left text).

`end`

  Align to the end edge of the text (right side in left-to-right text, left side in right-to-left text).

`left`

  Align to the left.

`right`

  Align to the right.

`center`

  Align to the center.

The `textBaseline` attribute's allowed keywords correspond to alignment points in the font:

The keywords map to these alignment points as follows:

`top`
  The top of the em square
`hanging`
  The hanging baseline
`middle`
  The middle of the em square
`alphabetic`
  The alphabetic baseline
`ideographic`
  The ideographic baseline
`bottom`
  The bottom of the em square

The `direction` attribute's allowed keywords are as follows:

`ltr`

  Treat input to the text preparation algorithm as left-to-right text.

`rtl`

  Treat input to the text preparation algorithm as right-to-left text.

`inherit`

  Default to the directionality of the `canvas` element or `Document` as appropriate.

The text preparation algorithm is as follows. It takes as input a string text, a `CanvasDrawingStyles` object target, and an optional length maxWidth. It returns an array of glyph shapes, each positioned on a common coordinate space, a physical alignment whose value is one of left, right, and center, and an inline box. (Most callers of this algorithm ignore the physical alignment and the inline box.)

1. If maxWidth was provided but is less than or equal to zero, return an empty array.

2. Replace all the space characters in text with U+0020 SPACE characters.

3. Let font be the current font of target, as given by that object's `font` attribute.

4. Apply the appropriate step from the following list to determine the value of direction:

   If the target object's `direction` attribute has the value "`ltr`"
     Let direction be 'ltr'.
   If the target object's `direction` attribute has the value "`rtl`"
     Let direction be 'rtl'.
   If the target object's font style source object is an element
     Let direction be the directionality of the target object's font style source object.
   If the target object's font style source object is a `Document` and that `Document` has a root element child
     Let direction be the directionality of the target object's font style source object's root element child.
   Otherwise
     Let direction be 'ltr'.

5. Form a hypothetical infinitely-wide CSS line box containing a single inline box containing the text text, with all the properties at their initial values except the 'font' property of the inline box set to font, the 'direction' property of the inline box set to direction, and the 'white-space' property set to 'pre'. [CSS]

6. If maxWidth was provided and the hypothetical width of the inline box in the hypothetical line box is greater than maxWidth CSS pixels, then change font to have a more condensed font (if one is available or if a reasonably readable one can be synthesized by applying a horizontal scale factor to the font) or a smaller font, and return to the previous step.

7. The anchor point is a point on the inline box, and the physical alignment is one of the values left, right, and center. These variables are determined by the `textAlign` and `textBaseline` values as follows:

   Horizontal position:

   If `textAlign` is `left`
   If `textAlign` is `start` and direction is 'ltr'
   If `textAlign` is `end` and direction is 'rtl'
     Let the anchor point's horizontal position be the left edge of the inline box, and let physical alignment be left.
   If `textAlign` is `right`
   If `textAlign` is `end` and direction is 'ltr'
   If `textAlign` is `start` and direction is 'rtl'
     Let the anchor point's horizontal position be the right edge of the inline box, and let physical alignment be right.
   If `textAlign` is `center`
     Let the anchor point's horizontal position be half way between the left and right edges of the inline box, and let physical alignment be center.

   Vertical position:

   If `textBaseline` is `top`
     Let the anchor point's vertical position be the top of the em box of the first available font of the inline box.
   If `textBaseline` is `hanging`
     Let the anchor point's vertical position be the hanging baseline of the first available font of the inline box.
   If `textBaseline` is `middle`
     Let the anchor point's vertical position be half way between the bottom and the top of the em box of the first available font of the inline box.
   If `textBaseline` is `alphabetic`
     Let the anchor point's vertical position be the alphabetic baseline of the first available font of the inline box.
   If `textBaseline` is `ideographic`
     Let the anchor point's vertical position be the ideographic baseline of the first available font of the inline box.
   If `textBaseline` is `bottom`
     Let the anchor point's vertical position be the bottom of the em box of the first available font of the inline box.

8. Let result be an array constructed by iterating over each glyph in the inline box from left to right (if any), adding to the array, for each glyph, the shape of the glyph as it is in the inline box, positioned on a coordinate space using CSS pixels with its origin is at the anchor point.

9. Return result, physical alignment, and the inline box.

###### 4.12.4.2.6 Building paths

Each object implementing the `CanvasPathMethods` interface has a path. A path has a list of zero or more subpaths. Each subpath consists of a list of one or more points, connected by straight or curved line segments, and a flag indicating whether the subpath is closed or not. A closed subpath is one where the last point of the subpath is connected to the first point of the subpath by a straight line. Subpaths with only one point are ignored when painting the path.

Paths have a need new subpath flag. When this flag is set, certain APIs create a new subpath rather than extending the previous one. When a path is created, its need new subpath flag must be set.

When an object implementing the `CanvasPathMethods` interface is created, its path must be initialised to zero subpaths.

context . `moveTo`(x, y)
path . `moveTo`(x, y)

  Creates a new subpath with the given point.

context . `closePath`()
path . `closePath`()

  Marks the current subpath as closed, and starts a new subpath with a point the same as the start and end of the newly closed subpath.

context . `lineTo`(x, y)
path . `lineTo`(x, y)

  Adds the given point to the current subpath, connected to the previous one by a straight line.

context . `quadraticCurveTo`(cpx, cpy, x, y)
path . `quadraticCurveTo`(cpx, cpy, x, y)

  Adds the given point to the current subpath, connected to the previous one by a quadratic Bézier curve with the given control point.

context . `bezierCurveTo`(cp1x, cp1y, cp2x, cp2y, x, y)
path . `bezierCurveTo`(cp1x, cp1y, cp2x, cp2y, x, y)

  Adds the given point to the current subpath, connected to the previous one by a cubic Bézier curve with the given control points.

context . `arcTo`(x1, y1, x2, y2, radiusX [, radiusY, rotation ] )
path . `arcTo`(x1, y1, x2, y2, radiusX [, radiusY, rotation ] )

  Adds an arc with the given control points and radius to the current subpath, connected to the previous point by a straight line.

  If two radii are provided, the first controls the width of the arc's ellipse, and the second controls the height. If only one is provided, or if they are the same, the arc is from a circle. In the case of an ellipse, the rotation argument controls the clockwise inclination of the ellipse relative to the x-axis.

  Throws an `IndexSizeError` exception if the given radius is negative.

context . `arc`(x, y, radius, startAngle, endAngle [, anticlockwise ] )
path . `arc`(x, y, radius, startAngle, endAngle [, anticlockwise ] )

  Adds points to the subpath such that the arc described by the circumference of the circle described by the arguments, starting at the given start angle and ending at the given end angle, going in the given direction (defaulting to clockwise), is added to the path, connected to the previous point by a straight line.

  Throws an `IndexSizeError` exception if the given radius is negative.

context . `ellipse`(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, anticlockwise] )
path . `ellipse`(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, anticlockwise] )

  Adds points to the subpath such that the arc described by the circumference of the ellipse described by the arguments, starting at the given start angle and ending at the given end angle, going in the given direction (defaulting to clockwise), is added to the path, connected to the previous point by a straight line.

  Throws an `IndexSizeError` exception if the given radius is negative.

context . `rect`(x, y, w, h)
path . `rect`(x, y, w, h)

  Adds a new closed subpath to the path, representing the given rectangle.

The following methods allow authors to manipulate the paths of objects implementing the `CanvasPathMethods` interface.

For `CanvasRenderingContext2D` objects, the points passed to the methods, and the resulting lines added to current default path by these methods, must be transformed according to the current transformation matrix before being added to the path.

The `moveTo(x, y)` method must create a new subpath with the specified point as its first (and only) point.

When the user agent is to ensure there is a subpath for a coordinate (x, y) on a path, the user agent must check to see if the path has its need new subpath flag set. If it does, the user agent must create a new subpath with the point (x, y) as its first (and only) point, as if the `moveTo()` method had been called, and must then unset the path's need new subpath flag.

The `closePath()` method must do nothing if the object's path has no subpaths. Otherwise, it must mark the last subpath as closed, create a new subpath whose first point is the same as the previous subpath's first point, and finally add this new subpath to the path.

If the last subpath had more than one point in its list of points, then this is equivalent to adding a straight line connecting the last point back to the first point, thus "closing" the shape, and then repeating the last (possibly implied) `moveTo()` call.

New points and the lines connecting them are added to subpaths using the methods described below. In all cases, the methods only modify the last subpath in the object's path.

The `lineTo(x, y)` method must ensure there is a subpath for (x, y) if the object's path has no subpaths. Otherwise, it must connect the last point in the subpath to the given point (x, y) using a straight line, and must then add the given point (x, y) to the subpath.

The `quadraticCurveTo(cpx, cpy, x, y)` method must ensure there is a subpath for (cpx, cpy), and then must connect the last point in the subpath to the given point (x, y) using a quadratic Bézier curve with control point (cpx, cpy), and must then add the given point (x, y) to the subpath. [BEZIER]

The `bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)` method must ensure there is a subpath for (cp1x, cp1y), and then must connect the last point in the subpath to the given point (x, y) using a cubic Bézier curve with control points (cp1x, cp1y) and (cp2x, cp2y). Then, it must add the point (x, y) to the subpath. [BEZIER]

The `arcTo(x1, y1, x2, y2, radiusX, radiusY, rotation)` method must first ensure there is a subpath for (x1, y1). Then, the behaviour depends on the arguments and the last point in the subpath, as described below.

Negative values for radiusX or radiusY must cause the implementation to throw an `IndexSizeError` exception. If radiusY is omitted, user agents must act as if it had the same value as radiusX.

Let the point (x0, y0) be the last point in the subpath, transformed by the inverse of the current transformation matrix (so that it is in the same coordinate system as the points passed to the method).

If the point (x0, y0) is equal to the point (x1, y1), or if the point (x1, y1) is equal to the point (x2, y2), or if both radiusX and radiusY are zero, then the method must add the point (x1, y1) to the subpath, and connect that point to the previous point (x0, y0) by a straight line.

Otherwise, if the points (x0, y0), (x1, y1), and (x2, y2) all lie on a single straight line, then the method must add the point (x1, y1) to the subpath, and connect that point to the previous point (x0, y0) by a straight line.

Otherwise, let The Arc be the shortest arc given by circumference of the ellipse that has radius radiusX on the major axis and radius radiusY on the minor axis, and whose semi-major axis is rotated rotation radians clockwise from the positive x-axis, and that has one point tangent to the half-infinite line that crosses the point (x0, y0) and ends at the point (x1, y1), and that has a different point tangent to the half-infinite line that ends at the point (x1, y1) and crosses the point (x2, y2). The points at which this ellipse touches these two lines are called the start and end tangent points respectively. The method must connect the point (x0, y0) to the start tangent point by a straight line, adding the start tangent point to the subpath, and then must connect the start tangent point to the end tangent point by The Arc, adding the end tangent point to the subpath.

The `arc(x, y, radius, startAngle, endAngle, anticlockwise)` and `ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle, anticlockwise)` methods draw arcs.

The `arc()` method is equivalent to the `ellipse()` method in the case where the two radii are equal. When the `arc()` method is invoked, it must act as if the `ellipse()` method had been invoked with the radiusX and radiusY arguments set to the value of the radius argument, the rotation argument set to zero, and the other arguments set to the same values as their identically named arguments on the `arc()` method.

When the `ellipse()` method is invoked, it must proceed as follows. First, if the object's path has any subpaths, then the method must add a straight line from the last point in the subpath to the start point of the arc. Then, it must add the start and end points of the arc to the subpath, and connect them with an arc. The arc and its start and end points are defined as follows:

Consider an ellipse that has its origin at (x, y), that has a major-axis radius radiusX and a minor-axis radius radiusY, and that is rotated about its origin such that its semi-major axis is inclined rotation radians clockwise from the x-axis.

If the anticlockwise argument is false and endAngle-startAngle is equal to or greater than 2π, or, if the anticlockwise argument is true and startAngle-endAngle is equal to or greater than 2π, then the arc is the whole circumference of this ellipse, and the point at startAngle along this circle's circumference, measured in radians clockwise from the ellipse's semi-major axis, acts as both the start point and the end point.

Otherwise, the points at startAngle and endAngle along this circle's circumference, measured in radians clockwise from the ellipse's semi-major axis, are the start and end points respectively, and the arc is the path along the circumference of this ellipse from the start point to the end point, going anti-clockwise if the anticlockwise argument is true, and clockwise otherwise. Since the points are on the ellipse, as opposed to being simply angles from zero, the arc can never cover an angle greater than 2π radians.

Even if the arc covers the entire circumference of the ellipse and there are no other points in the subpath, the path is not closed unless the `closePath()` method is appropriately invoked.

Negative values for radiusX or radiusY must cause the implementation to throw an `IndexSizeError` exception.

The `rect(x, y, w, h)` method must create a new subpath containing just the four points (x, y), (x+w, y), (x+w, y+h), (x, y+h), in that order, with those four points connected by straight lines, and must then mark the subpath as closed. It must then create a new subpath with the point (x, y) as the only point in the subpath.

###### 4.12.4.2.7 `Path2D` objects

`Path2D` objects can be used to declare paths that are then later used on `CanvasRenderingContext2D` objects. In addition to many of the APIs described in earlier sections, `Path2D` objects have methods to combine paths, and to add text to paths.

path = new `Path2D`()

  Creates a new empty `Path2D` object.

path = new `Path2D`(path)

  Creates a new `Path2D` object that is a copy of the argument.

path = new `Path2D`(paths [, fillRule ] )

  Creates a new `Path2D` object that describes a path that outlines the given paths, using the given fill rule.

path = new `Path2D`(d)

  Creates a new path with the path described by the argument, interpreted as SVG path data. [SVG]

path . `addPath`(path [, transform ] )
path . `addPathByStrokingPath`(path, styles [, transform ] )

  Adds to the path the path given by the argument.

  In the case of the stroking variants, the line styles are taken from the styles argument, which can be either a `DrawingStyle` object or a `CanvasRenderingContext2D` object.

path . `addText`(text, styles, transform, x, y [, maxWidth ] )
path . `addText`(text, styles, transform, path [, maxWidth ] )
path . `addPathByStrokingText`(text, styles, transform, x, y [, maxWidth ] )
path . `addPathByStrokingText`(text, styles, transform, path [, maxWidth ] )

  Adds to the path a series of subpaths corresponding to the given text. If the arguments give a coordinate, the text is drawn horizontally at the given coordinates. If the arguments give a path, the text is drawn along the path. If a maximum width is provided, the text will be scaled to fit that width if necessary.

  The font, and in the case of the stroking variants, the line styles, are taken from the styles argument, which can be either a `DrawingStyle` object or a `CanvasRenderingContext2D` object.

The `Path2D()` constructor, when invoked, must return a newly created `Path2D` object.

The `Path2D(path)` constructor, when invoked, must return a newly created `Path2D` object, to which the subpaths of the argument are added. (In other words, it returns a copy of the argument.)

The `Path2D(paths, fillRule)` constructor, when invoked, must run the following steps:

1. Run the appropriate step from the following list, based on the constructor's sectond argument:

   If it is "nonzero"

     Let merged path be a path that consists of a set of non-overlapping subpaths that exactly outline the points from which, in any of the paths provided in the constructor's first argument, the number of times a half-infinite straight line drawn from that point crosses a subpath going in one direction is not equal to the number of times it crosses a subpath going in the other direction.

   If it is "evenodd"

     Let merged path be a path that consists of a set of non-overlapping subpaths that exactly outline the points from which, in any of the paths provided in the constructor's first argument, the number of times a half-infinite straight line drawn from that point crosses that path is odd.

   The subpaths in merged path must be oriented such that for any point, the number of times a half-infinite straight line drawn from that point crosses a subpath is even if and only if the number of times a half-infinite straight line drawn from that same point crosses a subpath going in one direction is equal to the number of times it crosses a subpath going in the other direction.

2. Add all the subpaths in merged path to the `Path2D` object.

3. Set the `Path2D` object's need new subpath flag.

The `Path2D(d)` constructor must run the following steps:

1. Parse and interpret the d argument according to the SVG specification's rules for path data, thus obtaining an SVG path. [SVG]

   The resulting path could be empty. SVG defines error handling rules for parsing and applying path data.

2. Let (x, y) be the last point in the SVG path.

3. Create a new `Path2D` object and add all the subpaths in the SVG path, if any, to that `Path2D` object.

4. Create a new subpath in the `Path2D` object with (x, y) as the only point in the subpath.

5. Return the `Path2D` object as the constructed object.

The `addPath(b, transform)` method, when invoked on a `Path2D` object a, must run the following steps:

1. If the `Path2D` object b has no subpaths, abort these steps.

2. Create a copy of all the subpaths in b. Let this copy be known as c.

3. Transform all the coordinates and lines in c by the transform matrix transform, if it is not null.

4. Let (x, y) be the last point in the last subpath of c.

5. Add all the subpaths in c to a.

6. Create a new subpath in a with (x, y) as the only point in the subpath.

The `addPathByStrokingPath(b, styles, transform)` method, when invoked on a `Path2D` object a, must run the following steps:

1. If the `Path2D` object b has no subpaths, abort these steps.

2. Create a copy of all the subpaths in b. Let this copy be known as c.

3. Transform all the coordinates and lines in c by transformation matrix transform, if it is not null.

4. Let a new list of subpaths d be the result of tracing c, using the styles argument for the line styles.

5. Let (x, y) be the last point in the last subpath of d.

6. Add all the subpaths in d to a.

7. Create a new subpath in a with (x, y) as the only point in the subpath.

The `addText()` and `addPathByStrokingText()` methods each come in two variants: one rendering text at a given coordinate, and one rendering text along a given path. In both cases, the methods take a `CanvasDrawingStyles` object argument for the text and (if appropriate) line styles to use, an `SVGMatrix` object transform (which can be null), and a maximum width can optionally be provided.

When one of the `addText()` and `addPathByStrokingText()` variants that take as argument an (x, y) coordinate is invoked, the method must run the following algorithm:

1. Run the text preparation algorithm, passing it text, the `CanvasDrawingStyles` object argument, and, if the maxWidth argument was provided, that argument. Let glyphs be the result.

2. Move all the shapes in glyphs to the right by x CSS pixels and down by y CSS pixels.

3. Let glyph paths be a list of paths describing the shapes given in glyphs, with each CSS pixel in the coordinate space of glyphs mapped to one coordinate space unit in glyph paths. Subpaths in glyph paths must wind clockwise, regardless of how the user agent's font subsystem renders fonts and regardless of how the fonts themselves are defined.

4. Transform all the coordinates and lines in glyph paths by the transformation matrix transform, if it is not null.

5. If the method is `addPathByStrokingText()`, replace glyph paths by the result of tracing each path in glyph paths, using the `CanvasDrawingStyles` object argument for the line styles.

6. Let merged path be a path that consists of a set of non-overlapping subpaths that exactly outline the points from which, in any of the paths in glyph paths, the number of times a half-infinite straight line drawn from that point crosses that path is odd.

   The subpaths in merged path must be oriented such that for any point, the number of times a half-infinite straight line drawn from that point crosses a subpath is even if and only if the number of times a half-infinite straight line drawn from that same point crosses a subpath going in one direction is equal to the number of times it crosses a subpath going in the other direction.

   For example, suppose text consists of two overlapping glyphs "Q" and "p" (maybe the "Q" has a flourish that crosses into the tail of the "p"). The glyph paths therefore consist of two paths, each with two subpaths: one for the outside of the letter shape, and one for the inside of the letter shape. There are points that, according to the even-odd fill rule, are filled in both shapes simultaneously: where they overlap. As such, the subpaths from the two glyphs actually cross each other.

   The resulting merged path in such a situation would have just one path for these two letters, with a total of just three subpaths (one big outer outline, one for the inside of the "Q", and one for inside of the "p"). This single path would have no subpaths that cross each other.

7. Add all the subpaths in merged path to the `Path2D` object.

8. Set the `Path2D` object's need new subpath flag.

When one of the `addText()` and `addPathByStrokingText()` variants that take as argument a `Path2D` object is invoked, the method must run the following algorithm:

1. Let target be the `Path2D` object on which the method was invoked.

2. Let path be the `Path2D` object that was provided in the method's arguments.

3. Run the text preparation algorithm, passing it text, the `CanvasDrawingStyles` object argument, and, if the maxWidth argument was provided, that argument. Let glyphs be the resulting array, and physical alignment be the resulting alignment value.

4. Let width be the aggregate length of all the subpaths in path, including the distances from the last point of each closed subpath to the first point of that subpath.

5. Define L to be a linear coordinate line for of all the subpaths in path, with additional lines drawn between the last point and the first point of each closed subpath, such that the first point of the first subpath is defined as point 0, and the last point of the last subpath, if the last subpath is not closed, or the second occurrence first point of that subpath, if it is closed, is defined as point width.

6. Let offset be determined according to the appropriate step below:

   If physical alignment is left
     Let offset be zero.
   If physical alignment is right
     Let offset be width.
   If physical alignment is center
     Let offset be half of width.

7. Move all the shapes in glyphs to the right by offset CSS pixels.

8. Let transformed path list be an empty list of paths.

9. For each glyph glyph in the glyphs array, run these substeps:

   1. Let dx be the x-coordinate of the horizontal center of the bounding box of the shape described by glyph, in CSS pixels.

   2. If dx is negative or greater than width, skip the remainder of these substeps for this glyph.

   3. Recast dx to coordinate spaces units in path. (This just changes the dimensionality of dx, not its numeric value.)

   4. Find the point p on path (or implied closing lines in path) that corresponds to the position dx on the coordinate line L.

   5. Let θ be the clockwise angle from the positive x-axis to the side of the line that is tangential to path at the point p that is going in the same direction as the line at point p.

   6. Rotate the shape described by glyph clockwise by θ about the point that is at the dx coordinate horizontally and the zero coordinate vertically.

   7. Let (x, y) be the coordinate of the point p.

   8. Move the shape described by glyph to the right by x and down by y.

   9. Let glyph subpaths be a list of subpaths describing the shape given in glyph, with each CSS pixel in the coordinate space of glyph mapped to one coordinate space unit in glyph subpaths. Subpaths in glyph subpaths must wind clockwise, regardless of how the user agent's font subsystem renders fonts and regardless of how the fonts themselves are defined.

   10. Transform all the coordinates and lines in glyph subpaths by the transformation matrix transform, if it is not null.

   11. If the method is `addPathByStrokingText()`, replace glyph subpaths by the result of tracing glyph subpaths, using the `CanvasDrawingStyles` object argument for the line styles.

   12. Add all the subpaths in glyph subpaths to transformed path list.

10. Let merged path be a path that consists of a set of non-overlapping subpaths that exactly outline the points from which, in any of the paths in transformed path list, the number of times a half-infinite straight line drawn from that point crosses that path is odd.

    The subpaths in merged path must be oriented such that for any point, the number of times a half-infinite straight line drawn from that point crosses a subpath is even if and only if the number of times a half-infinite straight line drawn from that same point crosses a subpath going in one direction is equal to the number of times it crosses a subpath going in the other direction.

    See the equivalent step in the earlier algorithm for an example of this step. It's even more likely that there will be overlap with this method, since neighboring glyphs are likely to be rotated relative to each other.

11. Add all the subpaths in merged path to target.

12. Set the `Path2D` object's need new subpath flag.

###### 4.12.4.2.8 Transformations

Each `CanvasRenderingContext2D` object has a current transformation matrix, as well as methods (described in this section) to manipulate it. When a `CanvasRenderingContext2D` object is created, its transformation matrix must be initialised to the identity transform.

The transformation matrix is applied to coordinates when creating the current default path, and when painting text, shapes, and `Path2D` objects, on `CanvasRenderingContext2D` objects.

Most of the API uses `SVGMatrix` objects rather than this API. This API remains mostly for historical reasons.

The transformations must be performed in reverse order.

For instance, if a scale transformation that doubles the width is applied to the canvas, followed by a rotation transformation that rotates drawing operations by a quarter turn, and a rectangle twice as wide as it is tall is then drawn on the canvas, the actual result will be a square.

context . `currentTransform` [ = value ]

  Returns the transformation matrix, as an `SVGMatrix` object.

  Can be set, to change the transformation matrix.

context . `scale`(x, y)

  Changes the transformation matrix to apply a scaling transformation with the given characteristics.

context . `rotate`(angle)

  Changes the transformation matrix to apply a rotation transformation with the given characteristics. The angle is in radians.

context . `translate`(x, y)

  Changes the transformation matrix to apply a translation transformation with the given characteristics.

context . `transform`(a, b, c, d, e, f)

  Changes the transformation matrix to apply the matrix given by the arguments as described below.

context . `setTransform`(a, b, c, d, e, f)

  Changes the transformation matrix to the matrix given by the arguments as described below.

context . `resetTransform`()

  Changes the transformation matrix to the identity transform.

The `currentTransform`, on getting, must return the last object that it was set to. On setting, its value must be changed to the new value, and the transformation matrix must be updated to match the matrix described by the new value. When the `CanvasRenderingContext2D` object is created, the `currentTransform` attribute must be set a newly created `SVGMatrix` object. When the transformation matrix is mutated by the methods described in this section, the last `SVGMatrix` object to which the attribute has been set must be mutated in a corresponding fashion.

The `scale(x, y)` method must add the scaling transformation described by the arguments to the transformation matrix. The x argument represents the scale factor in the horizontal direction and the y argument represents the scale factor in the vertical direction. The factors are multiples.

The `rotate(angle)` method must add the rotation transformation described by the argument to the transformation matrix. The angle argument represents a clockwise rotation angle expressed in radians.

The `translate(x, y)` method must add the translation transformation described by the arguments to the transformation matrix. The x argument represents the translation distance in the horizontal direction and the y argument represents the translation distance in the vertical direction. The arguments are in coordinate space units.

The `transform(a, b, c, d, e, f)` method must replace the current transformation matrix with the result of multiplying the current transformation matrix with the matrix described by:

a | c | e
b | d | f
0 | 0 | 1

The arguments a, b, c, d, e, and f are sometimes called m11, m12, m21, m22, dx, and dy or m11, m21, m12, m22, dx, and dy. Care should be taken in particular with the order of the second and third arguments (b and c) as their order varies from API to API and APIs sometimes use the notation m12/m21 and sometimes m21/m12 for those positions.

The `setTransform(a, b, c, d, e, f)` method must reset the current transform to the identity matrix, and then invoke the `transform(a, b, c, d, e, f)` method with the same arguments.

The `resetTransform()` method must reset the current transform to the identity matrix.

###### 4.12.4.2.9 Image sources for 2D rendering contexts

Several methods in the `CanvasRenderingContext2D` API take the union type `CanvasImageSource` as an argument.

This union type allows objects implementing any of the following interfaces to be used as image sources:

- `HTMLImageElement` (`img` elements)
- `HTMLVideoElement` (`video` elements)
- `HTMLCanvasElement` (`canvas` elements)
- `CanvasRenderingContext2D`
- `ImageBitmap`

The `ImageBitmap` interface can be created from a number of other image-representing types, including `ImageData`.

When a user agent is required to check the usability of the image argument, where image is a `CanvasImageSource` object, the user agent must run these steps, which return either good, bad, or aborted:

1. If the image argument is an `HTMLImageElement` object that is in the broken state, then throw an `InvalidStateError` exception, return aborted, and abort these steps.

2. If the image argument is an `HTMLImageElement` object that is not fully decodable, or if the image argument is an `HTMLVideoElement` object whose `readyState` attribute is either `HAVE_NOTHING` or `HAVE_METADATA`, then return bad and abort these steps.

3. If the image argument is an `HTMLImageElement` object with an intrinsic width or intrinsic height (or both) equal to zero, then return bad and abort these steps.

4. If the image argument is an `HTMLCanvasElement` object with either a horizontal dimension or a vertical dimension equal to zero, then return bad and abort these steps.

5. Return good.

When a `CanvasImageSource` object represents an `HTMLImageElement`, the element's image must be used as the source image.

Specifically, when a `CanvasImageSource` object represents an animated image in an `HTMLImageElement`, the user agent must use the default image of the animation (the one that the format defines is to be used when animation is not supported or is disabled), or, if there is no such image, the first frame of the animation, when rendering the image for `CanvasRenderingContext2D` APIs.

When a `CanvasImageSource` object represents an `HTMLVideoElement`, then the frame at the current playback position when the method with the argument is invoked must be used as the source image when rendering the image for `CanvasRenderingContext2D` APIs, and the source image's dimensions must be the intrinsic width and intrinsic height of the media resource (i.e. after any aspect-ratio correction has been applied).

When a `CanvasImageSource` object represents an `HTMLCanvasElement`, the element's bitmap must be used as the source image.

When a `CanvasImageSource` object represents a `CanvasRenderingContext2D`, the object's scratch bitmap must be used as the source image.

When a `CanvasImageSource` object represents an element that is being rendered and that element has been resized, the original image data of the source image must be used, not the image as it is rendered (e.g. `width` and `height` attributes on the source element have no effect on how the object is interpreted when rendering the image for `CanvasRenderingContext2D` APIs).

When a `CanvasImageSource` object represents an `ImageBitmap`, the object's bitmap image data must be used as the source image.

The image argument is not origin-clean if it is an `HTMLImageElement` or `HTMLVideoElement` whose origin is not the same as the origin specified by the entry settings object, or if it is an `HTMLCanvasElement` whose bitmap's origin-clean flag is false, or if it is a `CanvasRenderingContext2D` object whose scratch bitmap's origin-clean flag is false.

###### 4.12.4.2.10 Fill and stroke styles

context . `fillStyle` [ = value ]

  Returns the current style used for filling shapes.

  Can be set, to change the fill style.

  The style can be either a string containing a CSS colour, or a `CanvasGradient` or `CanvasPattern` object. Invalid values are ignored.

context . `strokeStyle` [ = value ]

  Returns the current style used for stroking shapes.

  Can be set, to change the stroke style.

  The style can be either a string containing a CSS colour, or a `CanvasGradient` or `CanvasPattern` object. Invalid values are ignored.

The `fillStyle` attribute represents the colour or style to use inside shapes, and the `strokeStyle` attribute represents the colour or style to use for the lines around the shapes.

Both attributes can be either strings, `CanvasGradient`s, or `CanvasPattern`s. On setting, strings must be parsed as CSS <color> values and the colour assigned, and `CanvasGradient` and `CanvasPattern` objects must be assigned themselves. [CSSCOLOR] If the value is a string but cannot be parsed as a CSS <color> value, then it must be ignored, and the attribute must retain its previous value. If the new value is a `CanvasPattern` object that is marked as not origin-clean, then the scratch bitmap's origin-clean flag must be set to false.

When set to a `CanvasPattern` or `CanvasGradient` object, the assignment is live, meaning that changes made to the object after the assignment do affect subsequent stroking or filling of shapes.

On getting, if the value is a colour, then the serialisation of the colour must be returned. Otherwise, if it is not a colour but a `CanvasGradient` or `CanvasPattern`, then the respective object must be returned. (Such objects are opaque and therefore only useful for assigning to other attributes or for comparison to other gradients or patterns.)

The serialisation of a colour for a colour value is a string, computed as follows: if it has alpha equal to 1.0, then the string is a lowercase six-digit hex value, prefixed with a "#" character (U+0023 NUMBER SIGN), with the first two digits representing the red component, the next two digits representing the green component, and the last two digits representing the blue component, the digits being lowercase ASCII hex digits. Otherwise, the colour value has alpha less than 1.0, and the string is the colour value in the CSS `rgba()` functional-notation format: the literal string "`rgba`" (U+0072 U+0067 U+0062 U+0061) followed by a U+0028 LEFT PARENTHESIS, a base-ten integer in the range 0-255 representing the red component (using ASCII digits in the shortest form possible), a literal U+002C COMMA and U+0020 SPACE, an integer for the green component, a comma and a space, an integer for the blue component, another comma and space, a U+0030 DIGIT ZERO, if the alpha value is greater than zero then a U+002E FULL STOP (representing the decimal point), if the alpha value is greater than zero then one or more ASCII digits representing the fractional part of the alpha, and finally a U+0029 RIGHT PARENTHESIS. User agents must express the fractional part of the alpha value, if any, with the level of precision necessary for the alpha value, when reparsed, to be interpreted as the same alpha value.

When the context is created, the `fillStyle` and `strokeStyle` attributes must initially have the string value `#000000`.

When the value is a colour, it must not be affected by the transformation matrix when used to draw on bitmaps.

There are two types of gradients, linear gradients and radial gradients, both represented by objects implementing the opaque `CanvasGradient` interface.

Once a gradient has been created (see below), stops are placed along it to define how the colours are distributed along the gradient. The colour of the gradient at each stop is the colour specified for that stop. Between each such stop, the colours and the alpha component must be linearly interpolated over the RGBA space without premultiplying the alpha value to find the colour to use at that offset. Before the first stop, the colour must be the colour of the first stop. After the last stop, the colour must be the colour of the last stop. When there are no stops, the gradient is transparent black.

gradient . `addColorStop`(offset, color)

  Adds a colour stop with the given colour to the gradient at the given offset. 0.0 is the offset at one end of the gradient, 1.0 is the offset at the other end.

  Throws an `IndexSizeError` exception if the offset is out of range. Throws a `SyntaxError` exception if the colour cannot be parsed.

gradient = context . `createLinearGradient`(x0, y0, x1, y1)

  Returns a `CanvasGradient` object that represents a linear gradient that paints along the line given by the coordinates represented by the arguments.

gradient = context . `createRadialGradient`(x0, y0, r0, x1, y1, r1)

  Returns a `CanvasGradient` object that represents a radial gradient that paints along the cone given by the circles represented by the arguments.

  If either of the radii are negative, throws an `IndexSizeError` exception.

The `addColorStop(offset, color)` method on the `CanvasGradient` interface adds a new stop to a gradient. If the offset is less than 0 or greater than 1 then an `IndexSizeError` exception must be thrown. If the color cannot be parsed as a CSS <color> value, then a `SyntaxError` exception must be thrown. Otherwise, the gradient must have a new stop placed, at offset offset relative to the whole gradient, and with the colour obtained by parsing color as a CSS <color> value. If multiple stops are added at the same offset on a gradient, they must be placed in the order added, with the first one closest to the start of the gradient, and each subsequent one infinitesimally further along towards the end point (in effect causing all but the first and last stop added at each point to be ignored).

The `createLinearGradient(x0, y0, x1, y1)` method takes four arguments that represent the start point (x0, y0) and end point (x1, y1) of the gradient. The method must return a linear `CanvasGradient` initialised with the specified line.

Linear gradients must be rendered such that all points on a line perpendicular to the line that crosses the start and end points have the colour at the point where those two lines cross (with the colours coming from the interpolation and extrapolation described above). The points in the linear gradient must be transformed as described by the current transformation matrix when rendering.

If x0 = x1 and y0 = y1, then the linear gradient must paint nothing.

The `createRadialGradient(x0, y0, r0, x1, y1, r1)` method takes six arguments, the first three representing the start circle with origin (x0, y0) and radius r0, and the last three representing the end circle with origin (x1, y1) and radius r1. The values are in coordinate space units. If either of r0 or r1 are negative, an `IndexSizeError` exception must be thrown. Otherwise, the method must return a radial `CanvasGradient` initialised with the two specified circles.

Radial gradients must be rendered by following these steps:

1. If x0 = x1 and y0 = y1 and r0 = r1, then the radial gradient must paint nothing. Abort these steps.

2. Let x(ω) = (x1-x0)ω + x0

   Let y(ω) = (y1-y0)ω + y0

   Let r(ω) = (r1-r0)ω + r0

   Let the colour at ω be the colour at that position on the gradient (with the colours coming from the interpolation and extrapolation described above).

3. For all values of ω where r(ω) > 0, starting with the value of ω nearest to positive infinity and ending with the value of ω nearest to negative infinity, draw the circumference of the circle with radius r(ω) at position (x(ω), y(ω)), with the colour at ω, but only painting on the parts of the bitmap that have not yet been painted on by earlier circles in this step for this rendering of the gradient.

This effectively creates a cone, touched by the two circles defined in the creation of the gradient, with the part of the cone before the start circle (0.0) using the colour of the first offset, the part of the cone after the end circle (1.0) using the colour of the last offset, and areas outside the cone untouched by the gradient (transparent black).

The resulting radial gradient must then be transformed as described by the current transformation matrix when rendering.

Gradients must be painted only where the relevant stroking or filling effects requires that they be drawn.

Patterns are represented by objects implementing the opaque `CanvasPattern` interface.

pattern = context . `createPattern`(image, repetition)

  Returns a `CanvasPattern` object that uses the given image and repeats in the direction(s) given by the repetition argument.

  The allowed values for repetition are `repeat` (both directions), `repeat-x` (horizontal only), `repeat-y` (vertical only), and `no-repeat` (neither). If the repetition argument is empty, the value `repeat` is used.

  If the image isn't yet fully decoded, then nothing is drawn. If the image is a canvas with no data, throws an `InvalidStateError` exception.

pattern . `setTransform`(transform)

  Sets the transformation matrix that will be used when rendering the pattern during a fill or stroke painting operation.

To create objects of this type, the `createPattern(image, repetition)` method is used. When the method is invoked, the user agent must run the following steps:

1. Let image be the first argument and repetition be the second argument.

2. Check the usability of the image argument. If this returns aborted, then an exception has been thrown and the method doesn't return anything; abort these steps. If it returns bad, then return null and abort these steps. Otherwise it returns good; continue with these steps.

3. If repetition is the empty string, let it be "`repeat`".

4. If repetition is not a case-sensitive match for one of "`repeat`", "`repeat-x`", "`repeat-y`", or "`no-repeat`", throw a `SyntaxError` exception and abort these steps.

5. Create a new `CanvasPattern` object with the image image and the repetition behaviour given by repetition.

6. If the image argument is not origin-clean, then mark the `CanvasPattern` object as not origin-clean.

7. Return the `CanvasPattern` object.

Modifying the image used when creating a `CanvasPattern` object after calling the `createPattern()` method must not affect the pattern(s) rendered by the `CanvasPattern` object.

Patterns have a transformation matrix, which controls how the pattern is used when it is painted. Initially, a pattern's transformation matrix must be the identity transform.

When the `setTransform()` method is invoked on the pattern, the user agent must replace the pattern's transformation matrix with the one described by the `SVGMatrix` object provided as an argument to the method.

When a pattern is to be rendered within an area, the user agent must run the following steps to determine what is rendered:

1. Create an infinite transparent black bitmap.

2. Place a copy of the image on the bitmap, anchored such that its top left corner is at the origin of the coordinate space, with one coordinate space unit per CSS pixel of the image, then place repeated copies of this image horizontally to the left and right, if the repetition behaviour is "`repeat-x`", or vertically up and down, if the repetition behaviour is "`repeat-y`", or in all four directions all over the bitmap, if the repetition behaviour is "`repeat`".

   If the original image data is a bitmap image, the value painted at a point in the area of the repetitions is computed by filtering the original image data. When scaling up, if the `imageSmoothingEnabled` attribute is set to false, the image must be rendered using nearest-neighbor interpolation. Otherwise, the user agent may use any filtering algorithm (for example bilinear interpolation or nearest-neighbor). When such a filtering algorithm requires a pixel value from outside the original image data, it must instead use the value from wrapping the pixel's coordinates to the original image's dimensions. (That is, the filter uses 'repeat' behaviour, regardless of the value of the pattern's repetition behaviour.)

3. Transform the resulting bitmap according to the pattern's transformation matrix.

4. Transform the resulting bitmap again, this time according to the current transformation matrix.

5. Replace any part of the image outside the area in which the pattern is to be rendered with transparent black.

6. The resulting bitmap is what is to be rendered, with the same origin and same scale.

If a radial gradient or repeated pattern is used when the transformation matrix is singular, the resulting style must be transparent black (otherwise the gradient or pattern would be collapsed to a point or line, leaving the other pixels undefined). Linear gradients and solid colours always define all points even with singular tranformation matrices.

###### 4.12.4.2.11 Drawing rectangles to the bitmap

There are three methods that immediately draw rectangles to the bitmap. They each take four arguments; the first two give the x and y coordinates of the top left of the rectangle, and the second two give the width w and height h of the rectangle, respectively.

The current transformation matrix must be applied to the following four coordinates, which form the path that must then be closed to get the specified rectangle: (x, y), (x+w, y), (x+w, y+h), (x, y+h).

Shapes are painted without affecting the current default path, and are subject to the clipping region, and, with the exception of `clearRect()`, also shadow effects, global alpha, and global composition operators.

context . `clearRect`(x, y, w, h)

  Clears all pixels on the bitmap in the given rectangle to transparent black.

context . `fillRect`(x, y, w, h)

  Paints the given rectangle onto the bitmap, using the current fill style.

context . `strokeRect`(x, y, w, h)

  Paints the box that outlines the given rectangle onto the bitmap, using the current stroke style.

The `clearRect(x, y, w, h)` method must run the following steps:

1. Let pixels be the set of pixels in the specified rectangle that also intersect the current clipping region.

2. Clear the pixels in pixels to a fully transparent black, erasing any previous image.

3. Clear regions that cover the pixels in pixels on the scratch bitmap.

If either height or width are zero, this method has no effect, since the set of pixels would be empty.

The `fillRect(x, y, w, h)` method must paint the specified rectangular area using the `fillStyle`. If either height or width are zero, this method has no effect.

The `strokeRect(x, y, w, h)` method must take the result of tracing the path described below, using the `CanvasRenderingContext2D` object's line styles, and fill it with the `strokeStyle`.

If both w and h are zero, the path has a single subpath with just one point (x, y), and no lines, and this method thus has no effect (the trace a path algorithm returns an empty path in that case).

If just one of either w or h is zero, then the path has a single subpath consisting of two points, with coordinates (x, y) and (x+w, y+h), in that order, connected by a single straight line.

Otherwise, the path has a single subpath consisting of four points, with coordinates (x, y), (x+w, y), (x+w, y+h), and (x, y+h), connected to each other in that order by straight lines.

###### 4.12.4.2.12 Drawing text to the bitmap

context . `fillText`(text, x, y [, maxWidth ] )
context . `strokeText`(text, x, y [, maxWidth ] )

  Fills or strokes (respectively) the given text at the given position. If a maximum width is provided, the text will be scaled to fit that width if necessary.

metrics = context . `measureText`(text)

  Returns a `TextMetrics` object with the metrics of the given text in the current font.

metrics . `width`
metrics . `actualBoundingBoxLeft`
metrics . `actualBoundingBoxRight`
metrics . `fontBoundingBoxAscent`
metrics . `fontBoundingBoxDescent`
metrics . `actualBoundingBoxAscent`
metrics . `actualBoundingBoxDescent`
metrics . `emHeightAscent`
metrics . `emHeightDescent`
metrics . `hangingBaseline`
metrics . `alphabeticBaseline`
metrics . `ideographicBaseline`

  Returns the measurement described below.

The `CanvasRenderingContext2D` interface provides the following methods for rendering text.

The `fillText()` and `strokeText()` methods take three or four arguments, text, x, y, and optionally maxWidth, and render the given text at the given (x, y) coordinates ensuring that the text isn't wider than maxWidth if specified, using the current `font`, `textAlign`, and `textBaseline` values. Specifically, when the methods are called, the user agent must run the following steps:

1. Run the text preparation algorithm, passing it text, the `CanvasRenderingContext2D` object, and, if the maxWidth argument was provided, that argument. Let glyphs be the result.

2. Move all the shapes in glyphs to the right by x CSS pixels and down by y CSS pixels.

3. Paint the shapes given in glyphs, as transformed by the current transformation matrix, with each CSS pixel in the coordinate space of glyphs mapped to one coordinate space unit.

   For `fillText()`, `fillStyle` must be applied to the shapes and `strokeStyle` must be ignored. For `strokeText()`, the reverse holds: `strokeStyle` must be applied to the result of tracing the shapes using the `CanvasRenderingContext2D` object for the line styles, and `fillStyle` must be ignored.

   These shapes are painted without affecting the current path, and are subject to shadow effects, global alpha, the clipping region, and global composition operators.

4. If the text preparation algorithm used a font that has an origin that is not the same as the origin specified by the entry settings object (even if "using a font" means just checking if that font has a particular glyph in it before falling back to another font), then set the scratch bitmap's origin-clean flag to false.

The `measureText()` method takes one argument, text. When the method is invoked, the user agent must run the text preparation algorithm, passing it text and the `CanvasRenderingContext2D` object, and then using the returned inline box must create a new `TextMetrics` object with its attributes set as described in the following list. If doing these measurements requires using a font that has an origin that is not the same as that of the `Document` object that owns the `canvas` element (even if "using a font" means just checking if that font has a particular glyph in it before falling back to another font), then the method must throw a `SecurityError` exception. Otherwise, it must return the new `TextMetrics` object. [CSS]

`width` attribute

  The width of that inline box, in CSS pixels. (The text's advance width.)

`actualBoundingBoxLeft` attribute

  The distance parallel to the baseline from the alignment point given by the `textAlign` attribute to the left side of the bounding rectangle of the given text, in CSS pixels; positive numbers indicating a distance going left from the given alignment point.

  The sum of this value and the next (`actualBoundingBoxRight`) can be wider than the width of the inline box (`width`), in particular with slanted fonts where characters overhang their advance width.

`actualBoundingBoxRight` attribute

  The distance parallel to the baseline from the alignment point given by the `textAlign` attribute to the right side of the bounding rectangle of the given text, in CSS pixels; positive numbers indicating a distance going right from the given alignment point.

`fontBoundingBoxAscent` attribute

  The distance from the horizontal line indicated by the `textBaseline` attribute to the top of the highest bounding rectangle of all the fonts used to render the text, in CSS pixels; positive numbers indicating a distance going up from the given baseline.

  This value and the next are useful when rendering a background that must have a consistent height even if the exact text being rendered changes. The `actualBoundingBoxAscent` attribute (and its corresponding attribute for the descent) are useful when drawing a bounding box around specific text.

`fontBoundingBoxDescent` attribute

  The distance from the horizontal line indicated by the `textBaseline` attribute to the bottom of the lowest bounding rectangle of all the fonts used to render the text, in CSS pixels; positive numbers indicating a distance going down from the given baseline.

`actualBoundingBoxAscent` attribute

  The distance from the horizontal line indicated by the `textBaseline` attribute to the top of the bounding rectangle of the given text, in CSS pixels; positive numbers indicating a distance going up from the given baseline.

  This number can vary greatly based on the input text, even if the first font specified covers all the characters in the input. For example, the `actualBoundingBoxAscent` of a lowercase "o" from an alphabetic baseline would be less than that of an uppercase "F". The value can easily be negative; for example, the distance from the top of the em box (`textBaseline` value "`top`") to the top of the bounding rectangle when the given text is just a single comma "`,`" would likely (unless the font is quite unusual) be negative.

`actualBoundingBoxDescent` attribute

  The distance from the horizontal line indicated by the `textBaseline` attribute to the bottom of the bounding rectangle of the given text, in CSS pixels; positive numbers indicating a distance going down from the given baseline.

`emHeightAscent` attribute

  The distance from the horizontal line indicated by the `textBaseline` attribute to the highest top of the em squares in the line box, in CSS pixels; positive numbers indicating that the given baseline is below the top of that em square (so this value will usually be positive). Zero if the given baseline is the top of that em square; half the font size if the given baseline is the middle of that em square.

`emHeightDescent` attribute

  The distance from the horizontal line indicated by the `textBaseline` attribute to the lowest bottom of the em squares in the line box, in CSS pixels; positive numbers indicating that the given baseline is below the bottom of that em square (so this value will usually be negative). (Zero if the given baseline is the top of that em square.)

`hangingBaseline` attribute

  The distance from the horizontal line indicated by the `textBaseline` attribute to the hanging baseline of the line box, in CSS pixels; positive numbers indicating that the given baseline is below the hanging baseline. (Zero if the given baseline is the hanging baseline.)

`alphabeticBaseline` attribute

  The distance from the horizontal line indicated by the `textBaseline` attribute to the alphabetic baseline of the line box, in CSS pixels; positive numbers indicating that the given baseline is below the alphabetic baseline. (Zero if the given baseline is the alphabetic baseline.)

`ideographicBaseline` attribute

  The distance from the horizontal line indicated by the `textBaseline` attribute to the ideographic baseline of the line box, in CSS pixels; positive numbers indicating that the given baseline is below the ideographic baseline. (Zero if the given baseline is the ideographic baseline.)

Glyphs rendered using `fillText()` and `strokeText()` can spill out of the box given by the font size (the em square size) and the width returned by `measureText()` (the text width). Authors are encouraged to use the bounding box values described above if this is an issue.

A future version of the 2D context API may provide a way to render fragments of documents, rendered using CSS, straight to the canvas. This would be provided in preference to a dedicated way of doing multiline layout.

###### 4.12.4.2.13 Drawing paths to the canvas

The context always has a current default path. There is only one current default path, it is not part of the drawing state. The current default path is a path, as described above.

context . `beginPath`()

  Resets the current default path.

context . `fill`( [ fillRule ] )
context . `fill`(path [, fillRule ] )

  Fills the subpaths of the current default path or the given path with the current fill style, obeying the given fill rule.

context . `stroke`()
context . `stroke`(path)

  Strokes the subpaths of the current default path or the given path with the current stroke style.

context . `drawFocusIfNeeded`(element)
context . `drawFocusIfNeeded`(path, element)

  If the given element is focused, draws a focus ring around the current default path or the given path, following the platform conventions for focus rings.

context . `scrollPathIntoView`()
context . `scrollPathIntoView`(path)

  Scrolls the current default path or the given path into view. This is especially useful on devices with small screens, where the whole canvas might not be visible at once.

context . `clip`( [ fillRule ] )
context . `clip`(path [, fillRule ] )

  Further constrains the clipping region to the current default path or the given path, using the given fill rule to determine what points are in the path.

context . `resetClip`()

  Unconstrains the clipping region.

context . `isPointInPath`(x, y [, fillRule ] )
context . `isPointInPath`(path, x, y [, fillRule ] )

  Returns true if the given point is in the current default path or the given path, using the given fill rule to determine what points are in the path.

context . `isPointInStroke`(x, y)
context . `isPointInStroke`(path, x, y)

  Returns true if the given point would be in the region covered by the stroke of the current default path or the given path, given the current stroke style.

The `beginPath()` method must empty the list of subpaths in the context's current default path so that the it once again has zero subpaths.

Where the following method definitions use the term intended path, it means the `Path2D` argument, if one was provided, or the current default path otherwise.

When the intended path is a `Path2D` object, the coordinates and lines of its subpaths must be transformed according to the `CanvasRenderingContext2D` object's current transformation matrix when used by these methods (without affecting the `Path2D` object itself). When the intended path is the current default path, it is not affected by the transform. (This is because transformations already affect the current default path when it is constructed, so applying it when it is painted as well would result in a double transformation.)

The `fill()` method must fill all the subpaths of the intended path, using `fillStyle`, and using the fill rule indicated by the fillRule argument. Open subpaths must be implicitly closed when being filled (without affecting the actual subpaths).

The `stroke()` method must trace the intended path, using the `CanvasRenderingContext2D` object for the line styles, and then fill the resulting path using the `strokeStyle` attribute, using the non-zero winding rule.

As a result of how the algorithm to trace a path is defined, overlapping parts of the paths in one stroke operation are treated as if their union was what was painted.

The stroke style is affected by the transformation during painting, even if the intended path is the current default path.

Paths, when filled or stroked, must be painted without affecting the current default path or any `Path2D` objects, and must be subject to shadow effects, global alpha, the clipping region, and global composition operators. (The effect of transformations is described above and varies based on which path is being used.)

The `drawFocusIfNeeded(element)` method, when invoked, must run the following steps:

1. If element is not focused or is not a descendant of the element with whose context the method is associated, then abort these steps.

2. Draw a focus ring of the appropriate style along the intended path, following platform conventions.

   Some platforms only draw focus rings around elements that have been focused from the keyboard, and not those focused from the mouse. Other platforms simply don't draw focus rings around some elements at all unless relevant accessibility features are enabled. This API is intended to follow these conventions. User agents that implement distinctions based on the manner in which the element was focused are encouraged to classify focus driven by the `focus()` method based on the kind of user interaction event from which the call was triggered (if any).

   The focus ring should not be subject to the shadow effects, the global alpha, the global composition operators, the `fillStyle` attribute, the `strokeStyle` attribute, or any of the `CanvasDrawingStyles` members, but should be subject to the clipping region. (The effect of transformations is described above and varies based on which path is being used.)

3. Optionally, run the appropriate step from the following list:

   If the `CanvasRenderingContext2D` object's context bitmap mode is fixed

     Inform the user that the focus is at the location given by the intended path. User agents may wait until the next time the event loop reaches its update the rendering step to optionally inform the user.

   Otherwise

     Add instructions to the scratch bitmap's list of pending interface actions that inform the user that the focus is at the location of the bitmap given by the intended path.

User agents should not implicitly close open subpaths in the intended path when drawing the focus ring.

This might be a moot point, however. For example, if the focus ring is drawn as an axis-aligned bounding rectangle around the points in the intended path, then whether the subpaths are closed or not has no effect. This specification intentionally does not specify precisely how focus rings are to be drawn: user agents are expected to honor their platform's native conventions.

The `scrollPathIntoView()` method, when invoked, if the `CanvasRenderingContext2D` object's context bitmap mode is fixed, must run the following steps; and otherwise, must add instructions to the scratch bitmap's list of pending interface actions that run the following steps:

1. Let the specified rectangle be the rectangle of the bounding box of the intended path.

2. Let notional child be a hypothetical element that is a rendered child of the `canvas` element whose dimensions are those of the specified rectangle.

3. Scroll notional child into view with the align to top flag set.

4. Optionally, inform the user that the caret or selection (or both) cover the specified rectangle of the canvas. If the `CanvasRenderingContext2D` object's context bitmap mode was fixed when the method was invoked, the user agent may wait until the next time the event loop reaches its update the rendering step to optionally inform the user.

"Inform the user", as used in this section, does not imply any persistent state change. It could mean, for instance, calling a system accessibility API to notify assistive technologies such as magnification tools so that the user's magnifier moves to the given area of the canvas. However, it does not associate the path with the element, or provide a region for tactile feedback, etc. To persistently associate a region with information provided to accessibility tools, use the `addHitRegion()` API.

The `clip()` method must create a new clipping region by calculating the intersection of the current clipping region and the area described by the intended path, using the fill rule indicated by the fillRule argument. Open subpaths must be implicitly closed when computing the clipping region, without affecting the actual subpaths. The new clipping region replaces the current clipping region.

When the context is initialised, the clipping region must be set to the largest infinite surface (i.e. by default, no clipping occurs).

The `resetClip()` method must create a new clipping region that is the largest infinite surface. The new clipping region replaces the current clipping region.

The `isPointInPath()` method must return true if the point given by the x and y coordinates passed to the method, when treated as coordinates in the canvas coordinate space unaffected by the current transformation, is inside the intended path as determined by the fill rule indicated by the fillRule argument; and must return false otherwise. Open subpaths must be implicitly closed when computing the area inside the path, without affecting the actual subpaths. Points on the path itself must be considered to be inside the path. If either of the arguments is infinite or NaN, then the method must return false.

The `isPointInStroke()` method must return true if the point given by the x and y coordinates passed to the method, when treated as coordinates in the canvas coordinate space unaffected by the current transformation, is inside the path that results from tracing the intended path, using the non-zero winding rule, and using the `CanvasRenderingContext2D` object for the line styles; and must return false otherwise. Points on the resulting path must be considered to be inside the path. If either of the arguments is infinite or NaN, then the method must return false.

This `canvas` element has a couple of checkboxes. The path-related commands are highlighted:

    <canvas height=400 width=750>
     <label><input type=checkbox id=showA> Show As</label>
     <label><input type=checkbox id=showB> Show Bs</label>
     <!-- ... -->
    </canvas>
    <script>
     function drawCheckbox(context, element, x, y, paint) {
       context.save();
       context.font = '10px sans-serif';
       context.textAlign = 'left';
       context.textBaseline = 'middle';
       var metrics = context.measureText(element.labels[0].textContent);
       if (paint) {
         context.beginPath();
         context.strokeStyle = 'black';
         context.rect(x-5, y-5, 10, 10);
         context.stroke();
         context.addHitRegion({ control: element });
         if (element.checked) {
           context.fillStyle = 'black';
           context.fill();
         }
         context.fillText(element.labels[0].textContent, x+5, y);
       }
       context.beginPath();
       context.rect(x-7, y-7, 12 + metrics.width+2, 14);
       context.drawFocusIfNeeded(element)) {
       context.restore();
     }
     function drawBase() { /* ... */ }
     function drawAs() { /* ... */ }
     function drawBs() { /* ... */ }
     function redraw() {
       var canvas = document.getElementsByTagName('canvas')[0];
       var context = canvas.getContext('2d');
       context.clearRect(0, 0, canvas.width, canvas.height);
       drawCheckbox(context, document.getElementById('showA'), 20, 40, true);
       drawCheckbox(context, document.getElementById('showB'), 20, 60, true);
       drawBase();
       if (document.getElementById('showA').checked)
         drawAs();
       if (document.getElementById('showB').checked)
         drawBs();
     }
     function processClick(event) {
       var canvas = document.getElementsByTagName('canvas')[0];
       var context = canvas.getContext('2d');
       var x = event.clientX;
       var y = event.clientY;
       var node = event.target;
       while (node) {
         x -= node.offsetLeft - node.scrollLeft;
         y -= node.offsetTop - node.scrollTop;
         node = node.offsetParent;
       }
       drawCheckbox(context, document.getElementById('showA'), 20, 40, false);
       if (context.isPointInPath(x, y))
         document.getElementById('showA').checked = !(document.getElementById('showA').checked);
       drawCheckbox(context, document.getElementById('showB'), 20, 60, false);
       if (context.isPointInPath(x, y))
         document.getElementById('showB').checked = !(document.getElementById('showB').checked);
       redraw();
     }
     document.getElementsByTagName('canvas')[0].addEventListener('focus', redraw, true);
     document.getElementsByTagName('canvas')[0].addEventListener('blur', redraw, true);
     document.getElementsByTagName('canvas')[0].addEventListener('change', redraw, true);
     document.getElementsByTagName('canvas')[0].addEventListener('click', processClick, false);
     redraw();
    </script>

###### 4.12.4.2.14 Drawing images

To draw images, the `drawImage` method can be used.

This method can be invoked with three different sets of arguments:

- `drawImage(image, dx, dy)`
- `drawImage(image, dx, dy, dw, dh)`
- `drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh)`

context . `drawImage`(image, dx, dy)
context . `drawImage`(image, dx, dy, dw, dh)
context . `drawImage`(image, sx, sy, sw, sh, dx, dy, dw, dh)

  Draws the given image onto the canvas. The arguments are interpreted as follows:

  If the image isn't yet fully decoded, then nothing is drawn. If the image is a canvas with no data, throws an `InvalidStateError` exception.

When the `drawImage()` method is invoked, the user agent must run the following steps:

1. Check the usability of the image argument. If this returns aborted, then an exception has been thrown and the method doesn't return anything; abort these steps. If it returns bad, then abort these steps without drawing anything. Otherwise it returns good; continue with these steps.

2. Establish the source and destination rectangles as follows:

   If not specified, the dw and dh arguments must default to the values of sw and sh, interpreted such that one CSS pixel in the image is treated as one unit in the scratch bitmap's coordinate space. If the sx, sy, sw, and sh arguments are omitted, they must default to 0, 0, the image's intrinsic width in image pixels, and the image's intrinsic height in image pixels, respectively. If the image has no intrinsic dimensions, the concrete object size must be used instead, as determined using the CSS "Concrete Object Size Resolution" algorithm, with the specified size having neither a definite width nor height, nor any additional contraints, the object's intrinsic properties being those of the image argument, and the default object size being the size of the scratch bitmap. [CSSIMAGES]

   The source rectangle is the rectangle whose corners are the four points (sx, sy), (sx+sw, sy), (sx+sw, sy+sh), (sx, sy+sh).

   The destination rectangle is the rectangle whose corners are the four points (dx, dy), (dx+dw, dy), (dx+dw, dy+dh), (dx, dy+dh).

   When the source rectangle is outside the source image, the source rectangle must be clipped to the source image and the destination rectangle must be clipped in the same proportion.

   When the destination rectangle is outside the destination image (the scratch bitmap), the pixels that land outside the scratch bitmap are discarded, as if the destination was an infinite canvas whose rendering was clipped to the dimensions of the scratch bitmap.

3. If one of the sw or sh arguments is zero, abort these steps. Nothing is painted.

4. Paint the region of the image argument specified by the source rectangle on the region of the rendering context's scratch bitmap specified by the destination rectangle, after applying the current transformation matrix to the destination rectangle.

   The image data must be processed in the original direction, even if the dimensions given are negative.

   When scaling up, if the `imageSmoothingEnabled` attribute is set to true, the user agent should attempt to apply a smoothing algorithm to the image data when it is scaled. Otherwise, the image must be rendered using nearest-neighbor interpolation.

   This specification does not define the precise algorithm to use when scaling an image down, or when scaling an image up when the `imageSmoothingEnabled` attribute is set to true.

   When a `canvas` or `CanvasRenderingContext2D` object is drawn onto itself, the drawing model requires the source to be copied before the image is drawn, so it is possible to copy parts of a `canvas` or scratch bitmap onto overlapping parts of itself.

   If the original image data is a bitmap image, the value painted at a point in the destination rectangle is computed by filtering the original image data. The user agent may use any filtering algorithm (for example bilinear interpolation or nearest-neighbor). When the filtering algorithm requires a pixel value from outside the original image data, it must instead use the value from the nearest edge pixel. (That is, the filter uses 'clamp-to-edge' behaviour.) When the filtering algorithm requires a pixel value from outside the source rectangle but inside the original image data, then the value from the original image data must be used.

   Thus, scaling an image in parts or in whole will have the same effect. This does mean that when sprites coming from a single sprite sheet are to be scaled, adjacent images in the sprite sheet can interfere. This can be avoided by ensuring each sprite in the sheet is surrounded by a border of transparent black, or by copying sprites to be scaled into temporary `canvas` elements and drawing the scaled sprites from there.

   Images are painted without affecting the current path, and are subject to shadow effects, global alpha, the clipping region, and global composition operators.

5. If the image argument is not origin-clean, set the scratch bitmap's origin-clean flag to false.

###### 4.12.4.2.15 Hit regions

A hit region list is a list of hit regions for a bitmap.

Each hit region consists of the following information:

- A set of pixels on the bitmap for which this region is responsible.

- A bounding circumference on the bitmap that surrounds the hit region's set of pixels as they stood when it was created.

- Optionally, a non-empty string representing an ID for distinguishing the region from others.

- Optionally, a reference to another region that acts as the parent for this one.

- A count of regions that have this one as their parent, known as the hit region's child count.

- A cursor specification, in the form of either a CSS cursor value, or the string "`inherit`" meaning that the cursor of the hit region's parent, if any, or of the `canvas` element, if not, is to be used instead.

- Optionally, either a control, or an unbacked region description.

  A control is a reference to an `Element` node, to which, in certain conditions, the user agent will route events, and from which the user agent will determine the state of the hit region for the purposes of accessibility tools. (The control is ignored when it is not a descendant of the `canvas` element.)

  An unbacked region description consists of the following:

  - Optionally, a label.

  - An ARIA role, which, if the unbacked region description also has a label, could be the empty string.

context . `addHitRegion`(options)

  Adds a hit region to the bitmap. The argument is an object with the following members:

  `path` (default null)
    A `Path2D` object that describes the pixels that form part of the region. If this member is not provided or is set to null, the current default path is used instead.
  `fillRule` (default "`nonzero`")
    The fill rule to use when determining which pixels are inside the path.
  `id` (default empty string)
    The ID to use for this region. This is used in `MouseEvent` events on the `canvas` (`event.region`) and as a way to reference this region in later calls to `addHitRegion()`.
  `parentID` (default null)
    The ID of the parent region, for purposes of navigation by accessibility tools and for cursor fallback.
  `cursor` (default "`inherit`")
    The cursor to use when the mouse is over this region. The value "`inherit`" means to use the cursor for the parent region (as specified by the `parentID` member), if any, or to use the `canvas` element's cursor if the region has no parent.
  `control` (default null)
    An element (that is a descendant of the `canvas`) to which events are to be routed, and which accessibility tools are to use as a surrogate for describing and interacting with this region.
  `label` (default null)
    A text label for accessibility tools to use as a description of this region, if there is no control.
  `role` (default null)
    An ARIA role for accessibility tools to use to determine how to represent this region, if there is no control.

  Hit regions can be used for a variety of purposes:

  - With an ID, they can make hit detection easier by having the user agent check which region the mouse is over and include the ID in the mouse events.
  - With a control, they can make routing events to DOM elements automatic, allowing e.g. clicks on a `canvas` to automatically submit a form via a `button` element.
  - With a label, they can make it easier for users to explore a `canvas` without seeing it, e.g. by touch on a mobile device.
  - With a cursor, they can make it easier for different regions of the `canvas` to have different cursors, with the user agent automatically switching between them.
context . `removeHitRegion`(id)

  Removes a hit region (and all its descendants) from the canvas bitmap. The argument is the ID of a region added using `addHitRegion()`.

  The pixels that were covered by this region and its descendants are effectively cleared by this operation, leaving the regions non-interactive. In particular, regions that occupied the same pixels before the removed regions were added, overlapping them, do not resume their previous role.

context . `clearHitRegions`()

  Removes all hit regions from the canvas bitmap.

A hit region A is an ancestor region of a hit region B if B has a parent and its parent is either A or another hit region for which A is an ancestor region.

The region identified by the ID ID in a bitmap bitmap is the value returned by the following algorithm (which can return a hit region or nothing):

1. If ID is null, return nothing and abort these steps.

2. Let list be the hit region list associated with bitmap.

3. If there is a hit region in list whose ID is a case-sensitive match for ID, then return that hit region and abort these steps.

4. Otherwise, return nothing.

The region representing the control control for a bitmap bitmap is the value returned by the following algorithm (which can return a hit region or nothing):

1. Let list be the hit region list associated with bitmap.

2. If there is a hit region in list whose control is control, then return that hit region and abort these steps.

3. Otherwise, return nothing.

The control represented by a region region for a `canvas` element ancestor is the value returned by the following algorithm (which can return an element or nothing):

1. If region has no control, return nothing and abort these steps.

2. Let control be region's control.

3. If control is not a descendant of ancestor, then return nothing and abort these steps.

4. If control is no longer a supported interactive canvas fallback element, then return nothing and abort these steps.

5. Otherwise, return control.

The cursor for a hit region region of a `canvas` element ancestor is the value returned by the following algorithm:

1. Loop: If region has a cursor specification other than "`inherit`", then return that hit region's cursor specification and abort these steps.

2. If region has a parent, then let region be that hit region's parent, and return to the step labeled loop.

3. Otherwise, return the used value of the 'cursor' property for the `canvas` element, if any; if there isn't one, return 'auto'. [CSSUI]

The region for a pixel pixel on a bitmap bitmap is the value returned by the following algorithm (which can return a hit region or nothing):

1. Let list be the hit region list associated with bitmap.

2. If there is a hit region in list whose set of pixels contains pixel, then return that hit region and abort these steps.

3. Otherwise, return nothing.

To clear regions that cover the pixels pixels on a bitmap bitmap, the user agent must run the following steps:

1. Let list be the hit region list associated with bitmap.

2. Remove all pixels in pixels from the set of pixels of each hit region in list.

3. Garbage-collect the regions of bitmap.

To garbage-collect the regions of a bitmap bitmap, the user agent must run the following steps:

1. Let list be the hit region list associated with bitmap.

2. Loop: Let victim be the first hit region in list to have an empty set of pixels and a zero child count, if any. If there is no such hit region, abort these steps.

3. If victim has a parent, then decrement that hit region's child count by one.

4. Remove victim from list.

5. Jump back to the step labeled loop.

Adding a new region and calling `clearRect()` are the two ways this clearing algorithm can be invoked. The hit region list itself is also reset when the rendering context is reset, e.g. when a `CanvasRenderingContext2D` object is bound to or unbound from a `canvas`, or when the dimensions of the bitmap are changed.

An element is a supported interactive canvas fallback element if it is one of the following:

- an `a` element that represents a hyperlink and that does not have any `img` descendants

- a `button` element

- an `input` element whose `type` attribute is in one of the Checkbox or Radio Button states

- an `input` element that is a button but its `type` attribute is not in the Image Button state

- a `select` element with a `multiple` attribute or a display size greater than 1

- an `option` element that is in a list of options of a `select` element with a `multiple` attribute or a display size greater than 1

- a sorting interface `th` element

- an element that would not be interactive content except for having the `tabindex` attribute specified

- a non-interactive `table`, `caption`, `thead`, `tbody`, `tfoot`, `tr`, `td`, or `th` element

When the `addHitRegion()` method is invoked, the user agent must run the following steps:

1. Let arguments be the dictionary object provided as the method's argument.

2. If the arguments object's `path` member is not null, let source path be the `path` member's value. Otherwise, let it be the `CanvasRenderingContext2D` object's current default path.

3. Transform all the coordinates and lines in source path by the current transform matrix, if the arguments object's `path` member is not null.

4. Let specified pixels be the pixels contained in source path, using the fill rule indicated by the `fillRule` member.

5. Remove from specified pixels any pixels not contained within the clipping region.

6. If the arguments object's `id` member is the empty string, let it be null instead.

7. If the arguments object's `id` member is not null, then let previous region for this ID be the region identified by the ID given by the `id` member's value in this scratch bitmap, if any. If the `id` member is null or no such region currently exists, let previous region for this ID be null.

8. If the arguments object's `parentID` member is the empty string, let it be null instead.

9. If the arguments object's `parentID` member is not null, then let parent region be the region identified by the ID given by the `parentID` member's value in the scratch bitmap, if any. If the `parentID` member is null or no such region currently exists, let parent region be null.

10. If the arguments object's `label` member is the empty string, let it be null instead.

11. If any of the following conditions are met, throw a `NotSupportedError` exception and abort these steps.

    - The arguments object's `control` and `label` members are both non-null.

    - The arguments object's `control` and `role` members are both non-null.

    - The arguments object's `role` member's value is the empty string, and the `label` member's value is either null or the empty string.

    - The specified pixels has no pixels.

    - The arguments object's `control` member is neither null nor a supported interactive canvas fallback element.

    - The parent region is not null but has a control.

    - The previous region for this ID is the same hit region as the parent region.

    - The previous region for this ID is an ancestor region of the parent region.

12. If the `parentID` member is not null but parent region is null, then throw a `NotFoundError` exception and abort these steps.

13. If any of the following conditions are met, throw a `SyntaxError` exception and abort these steps.

    - The arguments object's `cursor` member is not null but is neither an ASCII case-insensitive match for the string "`inherit`", nor a valid CSS 'cursor' property value. [CSSUI]
    - The arguments object's `role` member is not null but its value is not an ordered set of unique space-separated tokens whose tokens are all case-sensitive matches for names of non-abstract WAI-ARIA roles. [ARIA]

14. Let region be a newly created hit region, with its information configured as follows:

    Hit region's set of pixels

      The specified pixels

    Hit region's bounding circumference

      A user-agent-defined shape that wraps the pixels contained in source path. (In the simplest case, this can just be the bounding rectangle; this specification allows it to be any shape in order to allow other interfaces.)

    Hit region's ID

      If the arguments object's `id` member is not null: the value of the `id` member. Otherwise, region has no id.

    Hit region's parent

      If parent region is not null: parent region. Otherwise, region has no parent.

    Hit region's child count

      Initially zero.

    Hit region's cursor specification

      If the arguments object's `cursor` member is not null: the value of the `cursor` member. Otherwise, the string "`inherit`".

    Hit region's control

      If the arguments object's `control` member is not null: the value of the `control` member. Otherwise, region has no control.

    Hit region's label

      If the arguments object's `label` member is not null: the value of the `label` member. Otherwise, region has no label.

    Hit region's ARIA role

      If the arguments object's `role` member is not null: the value of the `role` member (which might be the empty string). Otherwise, if the arguments object's `label` member is not null: the empty string. Otherwise, region has no ARIA role.

15. If the arguments object's `cursor` member is not null, then act as if a CSS rule for the `canvas` element setting its 'cursor' property had been seen, whose value was the hit region's cursor specification.

    For example, if the user agent prefetches cursor values, this would cause that to happen in response to an appropriately-formed `addHitRegion()` call.

16. If there is a previous region with this ID, remove it, and all hit regions for which it is an ancestor region, from the scratch bitmap's hit region list; then, if it had a parent region, decrement that hit region's child count by one.

17. If there is a parent region, increment its hit region's child count by one.

18. Clear regions that cover the pixels in region's set of pixels on this scratch bitmap.

19. Add region to the scratch bitmap's element's hit region list.

When the `removeHitRegion()` method is invoked, the user agent must run the following steps:

1. Let region be the region identified by the ID given by the method's argument in the rendering context's scratch bitmap. If no such region currently exists, abort these steps.

   If the method's argument is the empty string, then no region will match.

2. Remove region, and all hit regions for which it is an ancestor region, from the rendering context's scratch bitmap's hit region list; then, if it had a parent region, decrement that hit region's child count by one.

3. Garbage-collect the regions of the rendering context's scratch bitmap.

When the `clearHitRegions()` method is invoked, the user agent must remove all the hit regions from the rendering context's scratch bitmap's hit region list.

The `MouseEvent` interface is extended to support hit regions:

    partial interface MouseEvent {
      readonly attribute DOMString? region;
    };

    partial dictionary MouseEventInit {
      DOMString? region;
    };

event . `region`

  If the mouse was over a hit region, then this returns the hit region's ID, if it has one.

  Otherwise, returns null.

The `region` attribute on `MouseEvent` objects must return the value it was initialised to. When the object is created, this attribute must be initialised to null. It represents the hit region's ID if the mouse was over a hit region when the event was fired.

When a `MouseEvent` is to be fired at a `canvas` element by the user agent in response to a pointing device action other than a click (e.g. a `mousedown` event or a `mousemove` event), the user agent must run the canvas `MouseEvent` rerouting steps immediately prior to dispatching the event. This does not affect default actions (so for instance, if the event gets rerouted to an element that has a default action for `mousemove` events, this default action doesn't trigger).

Actual clicks are handled by the run authentic click activation steps, which also invoke these steps.

The canvas `MouseEvent` rerouting steps are as follows. If these steps say to act as normal, that means that the event must be fired as it would have had these requirements not been applied.

1. If the pointing device is not indicating a pixel on the `canvas`, then act as normal and abort these steps.

2. If the `canvas` element has no hit region list, then act as normal and abort these steps.

3. Let pixel be the pixel indicated by the pointing device.

4. Let region be the hit region that is the region for the pixel pixel on this `canvas` element's bitmap, if any.

5. If there is no region, then act as normal and abort these steps.

6. Let id be the region's ID, if any.

7. If there is an id, then initialise the event object's `region` attribute to id.

8. Let control be the control represented by region for this `canvas` element, if any.

9. If there is a control, then target the event object at control instead of the `canvas` element.

10. Continue dispatching the event, but with the updated event object and target as given in the above steps.

The `Touch` interface is extended to support hit regions also: [TOUCH]

    partial interface Touch {
      readonly attribute DOMString? region;
    };

touch . `region`

  If the touch point was over a hit region when it was first placed on the surface, then this returns the hit region's ID, if it has one.

  Otherwise, returns null.

The `region` attribute on a `Touch` object representing a touch point T must return the value obtained by running the following algorithm when T was first placed on the surface: [TOUCH]

1. If the touch point is not on a pixel on the `canvas`, then return null and abort these steps.

2. If the `canvas` element has no hit region list, then return null and abort these steps.

3. Let pixel be the pixel that the touch point is on.

4. Let region be the hit region that is the region for the pixel pixel on this `canvas` element's bitmap, if any.

5. If there is no region, then return null and abort these steps.

6. Let id be the region's ID, if any, or else null.

7. Return id.

When a user's pointing device cursor is positioned over a `canvas` element, user agents should render the pointing device cursor according to the cursor specification described by the cursor for the hit region that is the region for the pixel that the pointing device designates on the `canvas` element's bitmap.

User agents are encouraged to make use of the information present in a `canvas` element's hit region list to improve the accessibility of `canvas` elements.

Each hit region should be handled in a fashion equivalent to a node in a virtual DOM tree rooted at the `canvas` element. The hierarchy of this virtual DOM tree must match the hierarchy of the hit regions, as described by the parent of each region. Regions without a parent must be treated as children of the `canvas` element for the purpose of this virtual DOM tree. For each node in such a DOM tree, the hit region's bounding circumference gives the region of the screen to use when representing the node (if appropriate).

The semantics of a hit region for the purposes of this virtual DOM tree are those of the the control represented by the region, if it has one, or else of a non-interactive element whose ARIA role, if any, is that given by the hit region's ARIA role, and whose textual representation, if any, is given by the hit region's label.

For the purposes of accessibility tools, when an element C is a descendant of a `canvas` element and there is a region representing the control C for that `canvas` element's bitmap, then the element's position relative to the document should be presented as if it was that region in the `canvas` element's virtual DOM tree.

The semantics of a hit region for the purposes of this virtual DOM tree are those of the the control represented by the region, if it has one, or else of a non-interactive element whose ARIA role, if any, is that given by the hit region's ARIA role, and whose textual representation, if any, is given by the hit region's label.

Thus, for instance, a user agent on a touch-screen device could provide haptic feedback when the user croses over a hit region's bounding circumference, and then read the hit region's label to the user. Similarly, a desktop user agent with a virtual accessibility focus separate from the keyboard input focus could allow the user to navigate through the hit regions, using the virtual DOM tree described above to enable hierarchical navigation. When an interactive control inside the `canvas` element gains focus, if the control has a corresponding region, then that hit region's bounding circumference could be used to determine what area of the display to magnify.

###### 4.12.4.2.16 Pixel manipulation

imagedata = new `ImageData`(sw, sh)
imagedata = context . `createImageData`(sw, sh)

  Returns an `ImageData` object with the given dimensions. All the pixels in the returned object are transparent black.

  Throws an `IndexSizeError` exception if either of the width or height arguments are zero.

imagedata = context . `createImageData`(imagedata)

  Returns an `ImageData` object with the same dimensions as the argument. All the pixels in the returned object are transparent black.

imagedata = new `ImageData`(data, sw [, sh ] )

  Returns an `ImageData` object using the data provided in the `Uint8ClampedArray` argument, interpreted using the given dimensions.

  As each pixel in the data is represented by four numbers, the length of the data needs to be a multiple of four times the given width. If the height is provided as well, then the length needs to be exactly the width times the height times 4.

  Throws an `IndexSizeError` exception if the given data and dimensions can't be interpreted consistently, or if either dimension is zero.

imagedata = context . `getImageData`(sx, sy, sw, sh)

  Returns an `ImageData` object containing the image data for the given rectangle of the bitmap.

  Throws an `IndexSizeError` exception if the either of the width or height arguments are zero.

imagedata . `width`
imagedata . `height`

  Returns the actual dimensions of the data in the `ImageData` object, in pixels.

imagedata . `data`

  Returns the one-dimensional array containing the data in RGBA order, as integers in the range 0 to 255.

context . `putImageData`(imagedata, dx, dy [, dirtyX, dirtyY, dirtyWidth, dirtyHeight ] )

  Paints the data from the given `ImageData` object onto the bitmap. If a dirty rectangle is provided, only the pixels from that rectangle are painted.

  The `globalAlpha` and `globalCompositeOperation` attributes, as well as the shadow attributes, are ignored for the purposes of this method call; pixels in the canvas are replaced wholesale, with no composition, alpha blending, no shadows, etc.

  Throws a `NotSupportedError` exception if any of the arguments are not finite.

  Throws an `InvalidStateError` exception if the imagedata object's data has been neutered.

The `ImageData()` constructors and the `createImageData()` methods are used to instantiate new `ImageData` objects.

When the `ImageData()` constructor is invoked with two numeric arguments sw and sh, it must return a new `ImageData` object representing a transparent black rectangle with a width equal to sw and a height equal to sh, if both sw and sh are non-zero. If one or both of sw and sh are zero, then the constructor must throw an `IndexSizeError` exception instead.

When the `ImageData()` constructor is invoked with its first argument being an `Uint8ClampedArray` source and its second and (optionally) third argument(s) being numeric arguments sw and sh, it must run the following steps:

1. Let length be the number of bytes in source.

2. If length is not a non-zero integral multiple of four, throw an `InvalidStateError` exception and abort these steps.

3. Let length be length divided by four.

4. If length is not an integral multiple of sw, throw an `IndexSizeError` exception and abort these steps.

   At this step, the length is guaranteed to be greater than zero (otherwise the second step above would have aborted the steps), so if sw is zero, this step will throw the exception and abort these steps.

5. Let height be length divided by sw.

6. If the sh argument was not omitted, and its value is not equal to height, then throw an `IndexSizeError` exception and abort these steps.

7. Return a new `ImageData` object whose width is sw, whose height is height, and whose data is source.

   The resulting object's data is not a copy of source, it's the actual `Uint8ClampedArray` object passed as the first argument to the constructor.

When the `createImageData()` method is invoked with two numeric arguments sw and sh, it must return a new `ImageData` object representing a transparent black rectangle with a width equal to the absolute magnitude of sw and a height equal to the absolute magnitude of sh, if both sw and sh are non-zero. If one or both of sw and sh are zero, then the constructor must throw an `IndexSizeError` exception instead.

When the `createImageData()` method is invoked with a single imagedata argument, it must return a new `ImageData` object representing a transparent black rectangle with the same dimensions as the `ImageData` object passed as the argument.

The `getImageData(sx, sy, sw, sh)` method must, if either the sw or sh arguments are zero, throw an `IndexSizeError` exception; otherwise, if the scratch bitmap's origin-clean flag is set to false, it must throw a `SecurityError` exception; otherwise, it must return an `ImageData` object with width sw and height sh representing the scratch bitmap for the area of that bitmap denoted by the rectangle whose corners are the four points (sx, sy), (sx+sw, sy), (sx+sw, sy+sh), (sx, sy+sh), in the bitmap's coordinate space units. Pixels outside the scratch bitmap must be returned as transparent black. Pixels must be returned as non-premultiplied alpha values.

New `ImageData` objects must be initialised so that their `width` attribute is set to the number of pixels per row in the image data, their `height` attribute is set to the number of rows in the image data, and their `data` attribute, except where an existing array is provided, is initialised to a new `Uint8ClampedArray` object. The `Uint8ClampedArray` object must use a new Canvas Pixel `ArrayBuffer` for its storage, and must have a zero start offset and a length equal to the length of its storage, in bytes. The Canvas Pixel `ArrayBuffer` must contain the image data. At least one pixel's worth of image data must be returned. [ECMA262]

A Canvas Pixel `ArrayBuffer` is an `ArrayBuffer` whose data is represented in left-to-right order, row by row top to bottom, starting with the top left, with each pixel's red, green, blue, and alpha components being given in that order for each pixel. Each component of each pixel represented in this array must be in the range 0..255, representing the 8 bit value for that component. The components must be assigned consecutive indices starting with 0 for the top left pixel's red component. [ECMA262]

The `putImageData()` method writes data from `ImageData` structures back to the rendering context's scratch bitmap. Its arguments are: imagedata, dx, dy, dirtyX, dirtyY, dirtyWidth, and dirtyHeight.

When the last four arguments to this method are omitted, they must be assumed to have the values 0, 0, the `width` member of the imagedata structure, and the `height` member of the imagedata structure, respectively.

When invoked, the method must act as follows:

1. If the imagedata argument's `data` attribute has been neutered, throw an `InvalidStateError` exception and abort these steps.

2. If dirtyWidth is negative, let dirtyX be dirtyX+dirtyWidth, and let dirtyWidth be equal to the absolute magnitude of dirtyWidth.

   If dirtyHeight is negative, let dirtyY be dirtyY+dirtyHeight, and let dirtyHeight be equal to the absolute magnitude of dirtyHeight.

3. If dirtyX is negative, let dirtyWidth be dirtyWidth+dirtyX, and let dirtyX be zero.

   If dirtyY is negative, let dirtyHeight be dirtyHeight+dirtyY, and let dirtyY be zero.

4. If dirtyX+dirtyWidth is greater than the `width` attribute of the imagedata argument, let dirtyWidth be the value of that `width` attribute, minus the value of dirtyX.

   If dirtyY+dirtyHeight is greater than the `height` attribute of the imagedata argument, let dirtyHeight be the value of that `height` attribute, minus the value of dirtyY.

5. If, after those changes, either dirtyWidth or dirtyHeight is negative or zero, stop these steps without affecting any bitmaps.

6. For all integer values of x and y where dirtyX ≤ x < dirtyX+dirtyWidth and dirtyY ≤ y < dirtyY+dirtyHeight, copy the four channels of the pixel with coordinate (x, y) in the imagedata data structure's Canvas Pixel `ArrayBuffer` to the pixel with coordinate (dx+x, dy+y) in the rendering context's scratch bitmap.

The handling of pixel rounding when the specified coordinates are not integers is not defined by this specification, except that the following must result in no visible changes to the rendering:

    context.putImageData(context.getImageData(x, y, w, h), p, q);

...for any value of x, y, w, and h and where p is the smaller of x and the sum of x and w, and q is the smaller of y and the sum of y and h; and except that the following two calls:

    context.createImageData(w, h);
    context.getImageData(0, 0, w, h);

...must return `ImageData` objects with the same dimensions as each other, for any value of w and h. In other words, while user agents may round the arguments of these methods, any rounding performed must be performed consistently for all of the methods described in this section. (The constructors only work with integer values.)

Due to the lossy nature of converting to and from premultiplied alpha colour values, pixels that have just been set using `putImageData()` might be returned to an equivalent `getImageData()` as different values.

The current path, transformation matrix, shadow attributes, global alpha, the clipping region, and global composition operator must not affect the methods described in this section.

In the following example, the script generates an `ImageData` object so that it can draw onto it.

    // canvas is a reference to a <canvas> element
    var context = canvas.getContext('2d');

    // create a blank slate
    var data = context.createImageData(canvas.width, canvas.height);

    // create some plasma
    FillPlasma(data, 'green'); // green plasma

    // add a cloud to the plasma
    AddCloud(data, data.width/2, data.height/2); // put a cloud in the middle

    // paint the plasma+cloud on the canvas
    context.putImageData(data, 0, 0);

    // support methods
    function FillPlasma(data, color) { ... }
    function AddCloud(data, x, y) { ... }

Here is an example of using `getImageData()` and `putImageData()` to implement an edge detection filter.

    <!DOCTYPE HTML>
    <html>
     <head>
      <title>Edge detection demo</title>
      <script>
       var image = new Image();
       function init() {
         image.onload = demo;
         image.src = "image.jpeg";
       }
       function demo() {
         var canvas = document.getElementsByTagName('canvas')[0];
         var context = canvas.getContext('2d');

         // draw the image onto the canvas
         context.drawImage(image, 0, 0);

         // get the image data to manipulate
         var input = context.getImageData(0, 0, canvas.width, canvas.height);

         // get an empty slate to put the data into
         var output = context.createImageData(canvas.width, canvas.height);

         // alias some variables for convenience
         // notice that we are using input.width and input.height here
         // as they might not be the same as canvas.width and canvas.height
         // (in particular, they might be different on high-res displays)
         var w = input.width, h = input.height;
         var inputData = input.data;
         var outputData = output.data;

         // edge detection
         for (var y = 1; y < h-1; y += 1) {
           for (var x = 1; x < w-1; x += 1) {
             for (var c = 0; c < 3; c += 1) {
               var i = (y*w + x)*4 + c;
               outputData[i] = 127 + -inputData[i - w*4 - 4] -   inputData[i - w*4] - inputData[i - w*4 + 4] +
                                     -inputData[i - 4]       + 8*inputData[i]       - inputData[i + 4] +
                                     -inputData[i + w*4 - 4] -   inputData[i + w*4] - inputData[i + w*4 + 4];
             }
             outputData[(y*w + x)*4 + 3] = 255; // alpha
           }
         }

         // put the image data back after manipulation
         context.putImageData(output, 0, 0);
       }
      </script>
     </head>
     <body onload="init()">
      <canvas></canvas>
     </body>
    </html>

###### 4.12.4.2.17 Compositing

context . `globalAlpha` [ = value ]

  Returns the current alpha value applied to rendering operations.

  Can be set, to change the alpha value. Values outside of the range 0.0 .. 1.0 are ignored.

context . `globalCompositeOperation` [ = value ]

  Returns the current composition operation, from the values defined in the Compositing and Blending specification. [COMPOSITE].

  Can be set, to change the composition operation. Unknown values are ignored.

All drawing operations are affected by the global compositing attributes, `globalAlpha` and `globalCompositeOperation`.

The `globalAlpha` attribute gives an alpha value that is applied to shapes and images before they are composited onto the scratch bitmap. The value must be in the range from 0.0 (fully transparent) to 1.0 (no additional transparency). If an attempt is made to set the attribute to a value outside this range, including Infinity and Not-a-Number (NaN) values, the attribute must retain its previous value. When the context is created, the `globalAlpha` attribute must initially have the value 1.0.

The `globalCompositeOperation` attribute sets the current composition operator, which controls how shapes and images are drawn onto the scratch bitmap, once they have had `globalAlpha` and the current transformation matrix applied. The possible values are those defined in the Compositing and Blending specification, and include the values `source-over` and `copy`. [COMPOSITE]

These values are all case-sensitive — they must be used exactly as defined. User agents must not recognise values that are not a case-sensitive match for one of the values given in the Compositing and Blending specification. [COMPOSITE]

On setting, if the user agent does not recognise the specified value, it must be ignored, leaving the value of `globalCompositeOperation` unaffected. Otherwise, the attribute must be set to the given new value.

When the context is created, the `globalCompositeOperation` attribute must initially have the value `source-over`.

###### 4.12.4.2.18 Image smoothing

context . `imageSmoothingEnabled` [ = value ]

  Returns whether pattern fills and the `drawImage()` method will attempt to smooth images if their pixels don't line up exactly with the display, when scaling images up.

  Can be set, to change whether images are smoothed (true) or not (false).

The `imageSmoothingEnabled` attribute, on getting, must return the last value it was set to. On setting, it must be set to the new value. When the `CanvasRenderingContext2D` object is created, the attribute must be set to true.

###### 4.12.4.2.19 Shadows

All drawing operations are affected by the four global shadow attributes.

context . `shadowColor` [ = value ]

  Returns the current shadow colour.

  Can be set, to change the shadow colour. Values that cannot be parsed as CSS colours are ignored.

context . `shadowOffsetX` [ = value ]
context . `shadowOffsetY` [ = value ]

  Returns the current shadow offset.

  Can be set, to change the shadow offset. Values that are not finite numbers are ignored.

context . `shadowBlur` [ = value ]

  Returns the current level of blur applied to shadows.

  Can be set, to change the blur level. Values that are not finite numbers greater than or equal to zero are ignored.

The `shadowColor` attribute sets the colour of the shadow.

When the context is created, the `shadowColor` attribute initially must be fully-transparent black.

On getting, the serialisation of the colour must be returned.

On setting, the new value must be parsed as a CSS <color> value and the colour assigned. If the value cannot be parsed as a CSS <color> value then it must be ignored, and the attribute must retain its previous value. [CSSCOLOR]

The `shadowOffsetX` and `shadowOffsetY` attributes specify the distance that the shadow will be offset in the positive horizontal and positive vertical distance respectively. Their values are in coordinate space units. They are not affected by the current transformation matrix.

When the context is created, the shadow offset attributes must initially have the value 0.

On getting, they must return their current value. On setting, the attribute being set must be set to the new value, except if the value is infinite or NaN, in which case the new value must be ignored.

The `shadowBlur` attribute specifies the level of the blurring effect. (The units do not map to coordinate space units, and are not affected by the current transformation matrix.)

When the context is created, the `shadowBlur` attribute must initially have the value 0.

On getting, the attribute must return its current value. On setting the attribute must be set to the new value, except if the value is negative, infinite or NaN, in which case the new value must be ignored.

Shadows are only drawn if the opacity component of the alpha component of the colour of `shadowColor` is non-zero and either the `shadowBlur` is non-zero, or the `shadowOffsetX` is non-zero, or the `shadowOffsetY` is non-zero.

When shadows are drawn, they must be rendered as follows:

1. Let A be an infinite transparent black bitmap on which the source image for which a shadow is being created has been rendered.

2. Let B be an infinite transparent black bitmap, with a coordinate space and an origin identical to A.

3. Copy the alpha channel of A to B, offset by `shadowOffsetX` in the positive x direction, and `shadowOffsetY` in the positive y direction.

4. If `shadowBlur` is greater than 0:

   1. Let σ be half the value of `shadowBlur`.

   2. Perform a 2D Gaussian Blur on B, using σ as the standard deviation.

   User agents may limit values of σ to an implementation-specific maximum value to avoid exceeding hardware limitations during the Gaussian blur operation.

5. Set the red, green, and blue components of every pixel in B to the red, green, and blue components (respectively) of the colour of `shadowColor`.

6. Multiply the alpha component of every pixel in B by the alpha component of the colour of `shadowColor`.

7. The shadow is in the bitmap B, and is rendered as part of the drawing model described below.

If the current composition operation is `copy`, shadows effectively won't render (since the shape will overwrite the shadow).

###### 4.12.4.2.20 Drawing model

When a shape or image is painted, user agents must follow these steps, in the order given (or act as if they do):

1. Render the shape or image onto an infinite transparent black bitmap, creating image A, as described in the previous sections. For shapes, the current fill, stroke, and line styles must be honored, and the stroke must itself also be subjected to the current transformation matrix.

2. When shadows are drawn, render the shadow from image A, using the current shadow styles, creating image B.

3. When shadows are drawn, multiply the alpha component of every pixel in B by `globalAlpha`.

4. When shadows are drawn, composite B within the clipping region over the current scratch bitmap using the current composition operator.

5. Multiply the alpha component of every pixel in A by `globalAlpha`.

6. Composite A within the clipping region over the current scratch bitmap using the current composition operator.

When compositing onto the scratch bitmap, pixels that would fall outside of the scratch bitmap must be discarded.

###### 4.12.4.2.21 Best practices

When a canvas is interactive, authors should include focusable elements in the element's fallback content corresponding to each focusable part of the canvas, as in the example above.

To expose text and interactive content on a `canvas` to users of accessibility tools, authors should use the `addHitRegion()` API. When rendering focus rings, to ensure that focus rings have the appearance of native focus rings, authors should use the `drawFocusIfNeeded()` method, passing it the element for which a ring is being drawn. This method only draws the focus ring if the element is focused, so that it can simply be called whenever drawing the element, without checking whether the element is focused or not first.

In addition to drawing focus rings, authors should use the `scrollPathIntoView()` method when an element in the canvas is focused, to make sure it is visible on the screen (if applicable).

Authors should avoid implementing text editing controls using the `canvas` element. Doing so has a large number of disadvantages:

- Mouse placement of the caret has to be reimplemented.
- Keyboard movement of the caret has to be reimplemented (possibly across lines, for multiline text input).
- Scrolling of the text field has to be implemented (horizontally for long lines, vertically for multiline input).
- Native features such as copy-and-paste have to be reimplemented.
- Native features such as spell-checking have to be reimplemented.
- Native features such as drag-and-drop have to be reimplemented.
- Native features such as page-wide text search have to be reimplemented.
- Native features specific to the user, for example custom text services, have to be reimplemented. This is close to impossible since each user might have different services installed, and there is an unbounded set of possible such services.
- Bidirectional text editing has to be reimplemented.
- For multiline text editing, line wrapping has to be implemented for all relevant languages.
- Text selection has to be reimplemented.
- Dragging of bidirectional text selections has to be reimplemented.
- Platform-native keyboard shortcuts have to be reimplemented.
- Platform-native input method editors (IMEs) have to be reimplemented.
- Undo and redo functionality has to be reimplemented.
- Accessibility features such as magnification following the caret or selection have to be reimplemented.

This is a huge amount of work, and authors are most strongly encouraged to avoid doing any of it by instead using the `input` element, the `textarea` element, or the `contenteditable` attribute.

###### 4.12.4.2.22 Examples

This section is non-normative.

Here is an example of a script that uses canvas to draw pretty glowing lines.

    <canvas width="800" height="450"></canvas>
    <script>

     var context = document.getElementsByTagName('canvas')[0].getContext('2d');

     var lastX = context.canvas.width * Math.random();
     var lastY = context.canvas.height * Math.random();
     var hue = 0;
     function line() {
       context.save();
       context.translate(context.canvas.width/2, context.canvas.height/2);
       context.scale(0.9, 0.9);
       context.translate(-context.canvas.width/2, -context.canvas.height/2);
       context.beginPath();
       context.lineWidth = 5 + Math.random() * 10;
       context.moveTo(lastX, lastY);
       lastX = context.canvas.width * Math.random();
       lastY = context.canvas.height * Math.random();
       context.bezierCurveTo(context.canvas.width * Math.random(),
                             context.canvas.height * Math.random(),
                             context.canvas.width * Math.random(),
                             context.canvas.height * Math.random(),
                             lastX, lastY);

       hue = hue + 10 * Math.random();
       context.strokeStyle = 'hsl(' + hue + ', 50%, 50%)';
       context.shadowColor = 'white';
       context.shadowBlur = 10;
       context.stroke();
       context.restore();
     }
     setInterval(line, 50);

     function blank() {
       context.fillStyle = 'rgba(0,0,0,0.1)';
       context.fillRect(0, 0, context.canvas.width, context.canvas.height);
     }
     setInterval(blank, 40);

    </script>

The 2D rendering context for `canvas` is often used for sprite-based games. The following example demonstrates this:

Here is the source for this example:

    <!DOCTYPE HTML>
    <title>Blue Robot Demo</title>
    <base href="https://www.whatwg.org/demos/canvas/blue-robot/">
    <style>
      html { overflow: hidden; min-height: 200px; min-width: 380px; }
      body { height: 200px; position: relative; margin: 8px; }
      .buttons { position: absolute; bottom: 0px; left: 0px; margin: 4px; }
    </style>
    <canvas width="380" height="200"></canvas>
    <script>
     var Landscape = function (context, width, height) {
       this.offset = 0;
       this.width = width;
       this.advance = function (dx) {
         this.offset += dx;
       };
       this.horizon = height * 0.7;
       // This creates the sky gradient (from a darker blue to white at the bottom)
       this.sky = context.createLinearGradient(0, 0, 0, this.horizon);
       this.sky.addColorStop(0.0, 'rgb(55,121,179)');
       this.sky.addColorStop(0.7, 'rgb(121,194,245)');
       this.sky.addColorStop(1.0, 'rgb(164,200,214)');
       // this creates the grass gradient (from a darker green to a lighter green)
       this.earth = context.createLinearGradient(0, this.horizon, 0, height);
       this.earth.addColorStop(0.0, 'rgb(81,140,20)');
       this.earth.addColorStop(1.0, 'rgb(123,177,57)');
       this.paintBackground = function (context, width, height) {
         // first, paint the sky and grass rectangles
         context.fillStyle = this.sky;
         context.fillRect(0, 0, width, this.horizon);
         context.fillStyle = this.earth;
         context.fillRect(0, this.horizon, width, height-this.horizon);
         // then, draw the cloudy banner
         // we make it cloudy by having the draw text off the top of the
         // canvas, and just having the blurred shadow shown on the canvas
         context.save();
         context.translate(width-((this.offset+(this.width*3.2)) % (this.width*4.0))+0, 0);
         context.shadowColor = 'white';
         context.shadowOffsetY = 30+this.horizon/3; // offset down on canvas
         context.shadowBlur = '5';
         context.fillStyle = 'white';
         context.textAlign = 'left';
         context.textBaseline = 'top';
         context.font = '20px sans-serif';
         context.fillText('WHATWG ROCKS', 10, -30); // text up above canvas
         context.restore();
         // then, draw the background tree
         context.save();
         context.translate(width-((this.offset+(this.width*0.2)) % (this.width*1.5))+30, 0);
         context.beginPath();
         context.fillStyle = 'rgb(143,89,2)';
         context.lineStyle = 'rgb(10,10,10)';
         context.lineWidth = 2;
         context.rect(0, this.horizon+5, 10, -50); // trunk
         context.fill();
         context.stroke();
         context.beginPath();
         context.fillStyle = 'rgb(78,154,6)';
         context.arc(5, this.horizon-60, 30, 0, Math.PI*2); // leaves
         context.fill();
         context.stroke();
         context.restore();
       };
       this.paintForeground = function (context, width, height) {
         // draw the box that goes in front
         context.save();
         context.translate(width-((this.offset+(this.width*0.7)) % (this.width*1.1))+0, 0);
         context.beginPath();
         context.rect(0, this.horizon - 5, 25, 25);
         context.fillStyle = 'rgb(220,154,94)';
         context.lineStyle = 'rgb(10,10,10)';
         context.lineWidth = 2;
         context.fill();
         context.stroke();
         context.restore();
       };
     };
    </script>
    <script>
     var BlueRobot = function () {
       this.sprites = new Image();
       this.sprites.src = 'blue-robot.png'; // this sprite sheet has 8 cells
       this.targetMode = 'idle';
       this.walk = function () {
         this.targetMode = 'walk';
       };
       this.stop = function () {
         this.targetMode = 'idle';
       };
       this.frameIndex = {
         'idle': [0], // first cell is the idle frame
         'walk': [1,2,3,4,5,6], // the walking animation is cells 1-6
         'stop': [7], // last cell is the stopping animation
       };
       this.mode = 'idle';
       this.frame = 0; // index into frameIndex
       this.tick = function () {
         // this advances the frame and the robot
         // the return value is how many pixels the robot has moved
         this.frame += 1;
         if (this.frame >= this.frameIndex[this.mode].length) {
           // we've reached the end of this animation cycle
           this.frame = 0;
           if (this.mode != this.targetMode) {
             // switch to next cycle
             if (this.mode == 'walk') {
               // we need to stop walking before we decide what to do next
               this.mode = 'stop';
             } else if (this.mode == 'stop') {
               if (this.targetMode == 'walk')
                 this.mode = 'walk';
               else
                 this.mode = 'idle';
             } else if (this.mode == 'idle') {
               if (this.targetMode == 'walk')
                 this.mode = 'walk';
             }
           }
         }
         if (this.mode == 'walk')
           return 8;
         return 0;
       },
       this.paint = function (context, x, y) {
         if (!this.sprites.complete) return;
         // draw the right frame out of the sprite sheet onto the canvas
         // we assume each frame is as high as the sprite sheet
         // the x,y coordinates give the position of the bottom center of the sprite
         context.drawImage(this.sprites,
                           this.frameIndex[this.mode][this.frame] * this.sprites.height, 0, this.sprites.height, this.sprites.height,
                           x-this.sprites.height/2, y-this.sprites.height, this.sprites.height, this.sprites.height);
       };
     };
    </script>
    <script>
     var canvas = document.getElementsByTagName('canvas')[0];
     var context = canvas.getContext('2d');
     var landscape = new Landscape(context, canvas.width, canvas.height);
     var blueRobot = new BlueRobot();
     // paint when the browser wants us to, using requestAnimationFrame()
     function paint() {
       context.clearRect(0, 0, canvas.width, canvas.height);
       landscape.paintBackground(context, canvas.width, canvas.height);
       blueRobot.paint(context, canvas.width/2, landscape.horizon*1.1);
       landscape.paintForeground(context, canvas.width, canvas.height);
       requestAnimationFrame(paint);
     }
     paint();
     // but tick every 150ms, so that we don't slow down when we don't paint
     setInterval(function () {
       var dx = blueRobot.tick();
       landscape.advance(dx);
     }, 100);
    </script>
    <p class="buttons">
     <input type=button value="Walk" onclick="blueRobot.walk()">
     <input type=button value="Stop" onclick="blueRobot.stop()">
    <footer>
     <small> Blue Robot Player Sprite by <a href="http://johncolburn.deviantart.com/">JohnColburn</a>.
     Licensed under the terms of the Creative Commons Attribution Share-Alike 3.0 Unported license.</small>
     <small> This work is itself licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-sa/3.0/">Creative
     Commons Attribution-ShareAlike 3.0 Unported License</a>.</small>
    </footer>

##### 4.12.4.3 Colour spaces and colour correction

The `canvas` APIs must perform colour correction at only two points: when rendering images with their own gamma correction and colour space information onto a bitmap, to convert the image to the colour space used by the bitmaps (e.g. using the 2D Context's `drawImage()` method with an `HTMLImageElement` object), and when rendering the actual canvas bitmap to the output device.

Thus, in the 2D context, colours used to draw shapes onto the canvas will exactly match colours obtained through the `getImageData()` method.

The `toDataURL()` method must not include colour space information in the resources they return. Where the output format allows it, the colour of pixels in resources created by `toDataURL()` must match those returned by the `getImageData()` method.

In user agents that support CSS, the colour space used by a `canvas` element must match the colour space used for processing any colours for that element in CSS.

The gamma correction and colour space information of images must be handled in such a way that an image rendered directly using an `img` element would use the same colours as one painted on a `canvas` element that is then itself rendered. Furthermore, the rendering of images that have no colour correction information (such as those returned by the `toDataURL()` method) must be rendered with no colour correction.

Thus, in the 2D context, calling the `drawImage()` method to render the output of the `toDataURL()` method to the canvas, given the appropriate dimensions, has no visible effect.

##### 4.12.4.4 Serialising bitmaps to a file

When a user agent is to create a serialisation of the bitmap as a file, optionally with some given arguments, and optionally with a native flag set, it must create an image file in the format given by the first value of arguments, or, if there are no arguments, in the PNG format. [PNG]

If the native flag is set, or if the bitmap has one pixel per coordinate space unit, then the image file must have the same pixel data (before compression, if applicable) as the bitmap, and if the file format used supports encoding resolution metadata, the resolution of that bitmap (device pixels per coordinate space units being interpreted as image pixels per CSS pixel) must be given as well.

Otherwise, the image file's pixel data must be the bitmap's pixel data scaled to one image pixel per coordinate space unit, and if the file format used supports encoding resolution metadata, the resolution must be given as 96dpi (one image pixel per CSS pixel).

If arguments is not empty, the first value must be interpreted as a MIME type giving the format to use. If the type has any parameters, it must be treated as not supported.

For example, the value "`image/png`" would mean to generate a PNG image, the value "`image/jpeg`" would mean to generate a JPEG image, and the value "`image/svg+xml`" would mean to generate an SVG image (which would require that the user agent track how the bitmap was generated, an unlikely, though potentially awesome, feature).

User agents must support PNG ("`image/png`"). User agents may support other types. If the user agent does not support the requested type, it must create the file using the PNG format. [PNG]

User agents must convert the provided type to ASCII lowercase before establishing if they support that type.

For image types that do not support an alpha channel, the serialised image must be the bitmap image composited onto a solid black background using the source-over operator.

If the first argument in arguments gives a type corresponding to one of the types given in the first column of the following table, and the user agent supports that type, then the subsequent arguments, if any, must be treated as described in the second cell of that row.

Arguments for serialisation methods
Type | Other arguments | Reference
--- | --- | ---
`image/jpeg` | The second argument, if it is a number in the range 0.0 to 1.0 inclusive, must be treated as the desired quality level. If it is not a number or is outside that range, the user agent must use its default value, as if the argument had been omitted. | [JPEG]

For the purposes of these rules, an argument is considered to be a number if it is converted to an IDL double value by the rules for handling arguments of type `any` in the Web IDL specification. [WEBIDL]

Other arguments must be ignored and must not cause the user agent to throw an exception. A future version of this specification will probably define other parameters to be passed to these methods to allow authors to more carefully control compression settings, image metadata, etc.

##### 4.12.4.5 Security with `canvas` elements

This section is non-normative.

Information leakage can occur if scripts from one origin can access information (e.g. read pixels) from images from another origin (one that isn't the same).

To mitigate this, bitmaps used with `canvas` elements are defined to have a flag indicating whether they are origin-clean. All bitmaps start with their origin-clean set to true. The flag is set to false when cross-origin images or fonts are used.

The `toDataURL()`, `toBlob()`, and `getImageData()` methods check the flag and will throw a `SecurityError` exception rather than leak cross-origin data.

The flag can be reset in certain situations; for example, when a `CanvasRenderingContext2D` is bound to a new `canvas`, the bitmap is cleared and its flag reset.

### 4.13 Common idioms without dedicated elements

#### 4.13.1 The main part of the content

The main content of a page — not including headers and footers, navigation links, sidebars, advertisements, and so forth — can be marked up in a variety of ways, depending on the needs of the author.

The simplest solution is to not mark up the main content at all, and just leave it as implicit. Another way to think of this is that the `body` elements marks up the main content of the page, and the bits that aren't main content are excluded through the use of more appropriate elements like `aside` and `nav`.

Here is a short Web page marked up along this minimalistic school of thought. The main content is highlighted. Notice how all the other content in the `body` is marked up with elements to indicate that it's not part of the main content, in this case `header`, `nav`, and `footer`.

    <!DOCTYPE HTML>
    <html>
     <head>
      <title> My Toys </title>
     </head>
     <body>
      <header>
       <h1>My toys</h1>
      </header>
      <nav>
       <p><a href="/">Home</a></p>
       <p><a href="/contact">Contact</a></p>
      </nav>
      <p>I really like my chained book and my telephone. I'm not such a
      fan of my big ball.</p>
      <p>Another toy I like is my mirror.</p>
      <footer>
       <p>© copyright 2010 by the boy</p>
      </footer>
     </body>
    </html>

If the main content is an independent unit of content that one could imagine syndicating independently, then the `article` element would be appropriate to mark up the main content of the document.

The document in the previous example is here recast as a blog post:

    <!DOCTYPE HTML>
    <html>
     <head>
      <title> The Boy Blog: My Toys </title>
     </head>
     <body>
      <header>
       <h1>The Boy Blog</h1>
      </header>
      <nav>
       <p><a href="/">Home</a></p>
       <p><a href="/contact">Contact</a></p>
      </nav>
      <article>
       <header>
        <h1>My toys</h1>
        <p>Published August 4th</p>
       </header>
       <p>I really like my chained book and my telephone. I'm not such a
       fan of my big ball.</p>
       <p>Another toy I like is my mirror.</p>
      </article>
      <footer>
       <p>© copyright 2010 by the boy</p>
      </footer>
     </body>
    </html>

If the main content is not an independent unit of content so much as a section of a larger work, for instance a chapter, then the `section` element would be appropriate to mark up the main content of the document.

Here is the same document, but as a chapter in an online book:

    <!DOCTYPE HTML>
    <html>
     <head>
      <title> Chapter 2: My Toys — The Book of the Boy </title>
     </head>
     <body>
      <header>
       <hgroup>
        <h1>The Book of the Boy</h1>
        <h2>A book about boy stuff</h2>
       </hgroup>
      </header>
      <nav>
       <p><a href="/">Front Page</a></p>
       <p><a href="/toc">Table of Contents</a></p>
       <p><a href="/c1">Chapter 1</a> — <a href="/c3">Chapter 3</a></p>
      </nav>
      <section>
       <h1>Chapter 2: My Toys</h1>
       <p>I really like my chained book and my telephone. I'm not such a
       fan of my big ball.</p>
       <p>Another toy I like is my mirror.</p>
      </section>
     </body>
    </html>

If neither `article` nor `section` would be appropriate, but the main content still needs an explicit element, for example for styling purposes, then the `main` element can be used.

This is the same as the original example, but using `main` for the main content instead of leaving it implied:

    <!DOCTYPE HTML>
    <html>
     <head>
      <title> My Toys </title>
      <style>
       body > main { background: navy; color: yellow; }
      </style>
     </head>
     <body>
      <header>
       <h1>My toys</h1>
      </header>
      <nav>
       <p><a href="/">Home</a></p>
       <p><a href="/contact">Contact</a></p>
      </nav>
      <main>
       <p>I really like my chained book and my telephone. I'm not such a
       fan of my big ball.</p>
       <p>Another toy I like is my mirror.</p>
      </main>
      <footer>
       <p>© copyright 2010 by the boy</p>
      </footer>
     </body>
    </html>

#### 4.13.2 Bread crumb navigation

This specification does not provide a machine-readable way of describing bread-crumb navigation menus. Authors are encouraged to just use a series of links in a paragraph. The `nav` element can be used to mark the section containing these paragraphs as being navigation blocks.

In the following example, the current page can be reached via two paths.

    <nav>
     <p>
      <a href="/">Main</a> ▸
      <a href="/products/">Products</a> ▸
      <a href="/products/dishwashers/">Dishwashers</a> ▸
      <a>Second hand</a>
     </p>
     <p>
      <a href="/">Main</a> ▸
      <a href="/second-hand/">Second hand</a> ▸
      <a>Dishwashers</a>
     </p>
    </nav>

#### 4.13.3 Tag clouds

This specification does not define any markup specifically for marking up lists of keywords that apply to a group of pages (also known as tag clouds). In general, authors are encouraged to either mark up such lists using `ul` elements with explicit inline counts that are then hidden and turned into a presentational effect using a style sheet, or to use SVG.

Here, three tags are included in a short tag cloud:

    <style>
    @media screen, print, handheld, tv {
      /* should be ignored by non-visual browsers */
      .tag-cloud > li > span { display: none; }
      .tag-cloud > li { display: inline; }
      .tag-cloud-1 { font-size: 0.7em; }
      .tag-cloud-2 { font-size: 0.9em; }
      .tag-cloud-3 { font-size: 1.1em; }
      .tag-cloud-4 { font-size: 1.3em; }
      .tag-cloud-5 { font-size: 1.5em; }
    }
    </style>
    ...
    <ul class="tag-cloud">
     <li class="tag-cloud-4"><a title="28 instances" href="/t/apple">apple</a> <span>(popular)</span>
     <li class="tag-cloud-2"><a title="6 instances"  href="/t/kiwi">kiwi</a> <span>(rare)</span>
     <li class="tag-cloud-5"><a title="41 instances" href="/t/pear">pear</a> <span>(very popular)</span>
    </ul>

The actual frequency of each tag is given using the `title` attribute. A CSS style sheet is provided to convert the markup into a cloud of differently-sized words, but for user agents that do not support CSS or are not visual, the markup contains annotations like "(popular)" or "(rare)" to categorise the various tags by frequency, thus enabling all users to benefit from the information.

The `ul` element is used (rather than `ol`) because the order is not particularly important: while the list is in fact ordered alphabetically, it would convey the same information if ordered by, say, the length of the tag.

The `tag` `rel`-keyword is not used on these `a` elements because they do not represent tags that apply to the page itself; they are just part of an index listing the tags themselves.

#### 4.13.4 Conversations

This specification does not define a specific element for marking up conversations, meeting minutes, chat transcripts, dialogues in screenplays, instant message logs, and other situations where different players take turns in discourse.

Instead, authors are encouraged to mark up conversations using `p` elements and punctuation. Authors who need to mark the speaker for styling purposes are encouraged to use `span` or `b`. Paragraphs with their text wrapped in the `i` element can be used for marking up stage directions.

This example demonstrates this using an extract from Abbot and Costello's famous sketch, Who's on first:

    <p> Costello: Look, you gotta first baseman?
    <p> Abbott: Certainly.
    <p> Costello: Who's playing first?
    <p> Abbott: That's right.
    <p> Costello becomes exasperated.
    <p> Costello: When you pay off the first baseman every month, who gets the money?
    <p> Abbott: Every dollar of it.

The following extract shows how an IM conversation log could be marked up, using the `data` element to provide Unix timestamps for each line. Note that the timestamps are provided in a format that the `time` element does not support, so the `data` element is used instead (namely, Unix `time_t` timestamps). Had the author wished to mark up the data using one of the date and time formats supported by the `time` element, that element could have been used instead of `data`. This could be advantageous as it would allow data analysis tools to detect the timestamps unambiguously, without coordination with the page author.

    <p> <data value="1319898155">14:22</data> <b>egof</b> I'm not that nerdy, I've only seen 30% of the star trek episodes
    <p> <data value="1319898192">14:23</data> <b>kaj</b> if you know what percentage of the star trek episodes you have seen, you are inarguably nerdy
    <p> <data value="1319898200">14:23</data> <b>egof</b> it's unarguably
    <p> <data value="1319898228">14:23</data> <i>* kaj blinks</i>
    <p> <data value="1319898260">14:24</data> <b>kaj</b> you are not helping your case

HTML does not have a good way to mark up graphs, so descriptions of interactive conversations from games are more difficult to mark up. This example shows one possible convention using `dl` elements to list the possible responses at each point in the conversation. Another option to consider is describing the conversation in the form of a DOT file, and outputting the result as an SVG image to place in the document. [DOT]

    <p> Next, you meet a fisherman. You can say one of several greetings:
    <dl>
     <dt> "Hello there!"
     <dd>
      <p> He responds with "Hello, how may I help you?"; you can respond with:
      <dl>
       <dt> "I would like to buy a fish."
       <dd> <p> He sells you a fish and the conversation finishes.
       <dt> "Can I borrow your boat?"
       <dd>
        <p> He is surprised and asks "What are you offering in return?".
        <dl>
         <dt> "Five gold." (if you have enough)
         <dt> "Ten gold." (if you have enough)
         <dt> "Fifteen gold." (if you have enough)
         <dd> <p> He lends you his boat. The conversation ends.
         <dt> "A fish." (if you have one)
         <dt> "A newspaper." (if you have one)
         <dt> "A pebble." (if you have one)
         <dd> <p> "No thanks", he replies. Your conversation options
         at this point are the same as they were after asking to borrow
         his boat, minus any options you've suggested before.
        </dl>
       </dd>
      </dl>
     </dd>
     <dt> "Vote for me in the next election!"
     <dd> <p> He turns away. The conversation finishes.
     <dt> "Sir, are you aware that your fish are running away?"
     <dd>
      <p> He looks at you skeptically and says "Fish cannot run, sir".
      <dl>
       <dt> "You got me!"
       <dd> <p> The fisherman sighs and the conversation ends.
       <dt> "Only kidding."
       <dd> <p> "Good one!" he retorts. Your conversation options at this
       point are the same as those following "Hello there!" above.
       <dt> "Oh, then what are they doing?"
       <dd> <p> He looks at his fish, giving you an opportunity to steal
       his boat, which you do. The conversation ends.
      </dl>
     </dd>
    </dl>

In some games, conversations are simpler: each character merely has a fixed set of lines that they say. In this example, a game FAQ/walkthrough lists some of the known possible responses for each character:

    <section>
     <h1>Dialogue</h1>
     <p><small>Some characters repeat their lines in order each time you interact
     with them, others randomly pick from amongst their lines. Those who respond in
     order have numbered entries in the lists below.</small>
     <h2>The Shopkeeper</h2>
     <ul>
      <li>How may I help you?
      <li>Fresh apples!
      <li>A loaf of bread for madam?
     </ul>
     <h2>The pilot</h2>
     <p>Before the accident:
     <ul>
      </li>I'm about to fly out, sorry!
      </li>Sorry, I'm just waiting for flight clearance and then I'll be off!
     </ul>
     <p>After the accident:
     <ol>
      <li>I'm about to fly out, sorry!
      <li>Ok, I'm not leaving right now, my plane is being cleaned.
      <li>Ok, it's not being cleaned, it needs a minor repair first.
      <li>Ok, ok, stop bothering me! Truth is, I had a crash.
     </ol>
     <h2>Clan Leader</h2>
     <p>During the first clan meeting:
     <ul>
      <li>Hey, have you seen my daughter? I bet she's up to something nefarious again...
      <li>Nice weather we're having today, eh?
      <li>The name is Bailey, Jeff Bailey. How can I help you today?
      <li>A glass of water? Fresh from the well!
     </ul>
     <p>After the earthquake:
     <ol>
      <li>Everyone is safe in the shelter, we just have to put out the fire!
      <li>I'll go and tell the fire brigade, you keep hosing it down!
     </ol>
    </section>

#### 4.13.5 Footnotes

HTML does not have a dedicated mechanism for marking up footnotes. Here are the suggested alternatives.

For short inline annotations, the `title` attribute could be used.

In this example, two parts of a dialogue are annotated with footnote-like content using the `title` attribute.

    <p> <b>Customer</b>: Hello! I wish to register a complaint. Hello. Miss?
    <p> <b>Shopkeeper</b>: <span title="Colloquial pronunciation of 'What do you'"
    >Watcha</span> mean, miss?
    <p> <b>Customer</b>: Uh, I'm sorry, I have a cold. I wish to make a complaint.
    <p> <b>Shopkeeper</b>: Sorry, <span title="This is, of course, a lie.">we're
    closing for lunch</span>.

Unfortunately, relying on the `title` attribute is currently discouraged as many user agents do not expose the attribute in an accessible manner as required by this specification (e.g. requiring a pointing device such as a mouse to cause a tooltip to appear, which excludes keyboard-only users and touch-only users, such as anyone with a modern phone or tablet).

If the `title` attribute is used, CSS can used to draw the reader's attention to the elements with the attribute.

For example, the following CSS places a dashed line below elements that have a `title` attribute.

    [title] { border-bottom: thin dashed; }

For longer annotations, the `a` element should be used, pointing to an element later in the document. The convention is that the contents of the link be a number in square brackets.

In this example, a footnote in the dialogue links to a paragraph below the dialogue. The paragraph then reciprocally links back to the dialogue, allowing the user to return to the location of the footnote.

    <p> Announcer: Number 16: The <i>hand</i>.
    <p> Interviewer: Good evening. I have with me in the studio tonight
    Mr Norman St John Polevaulter, who for the past few years has been
    contradicting people. Mr Polevaulter, why <em>do</em> you
    contradict people?
    <p> Norman: I don't. <sup><a href="#fn1" id="r1">[1]</a></sup>
    <p> Interviewer: You told me you did!
    ...
    <section>
     <p id="fn1"><a href="#r1">[1]</a> This is, naturally, a lie,
     but paradoxically if it were true he could not say so without
     contradicting the interviewer and thus making it false.</p>
    </section>

For side notes, longer annotations that apply to entire sections of the text rather than just specific words or sentences, the `aside` element should be used.

In this example, a sidebar is given after a dialogue, giving it some context.

    <p> <span class="speaker">Customer</span>: I will not buy this record, it is scratched.
    <p> <span class="speaker">Shopkeeper</span>: I'm sorry?
    <p> <span class="speaker">Customer</span>: I will not buy this record, it is scratched.
    <p> <span class="speaker">Shopkeeper</span>: No no no, this's'a tobacconist's.
    <aside>
     <p>In 1970, the British Empire lay in ruins, and foreign
     nationalists frequented the streets — many of them Hungarians
     (not the streets — the foreign nationals). Sadly, Alexander
     Yalt has been publishing incompetently-written phrase books.
    </aside>

For figures or tables, footnotes can be included in the relevant `figcaption` or `caption` element, or in surrounding prose.

In this example, a table has cells with footnotes that are given in prose. A `figure` element is used to give a single legend to the combination of the table and its footnotes.

    <figure>
     <figcaption>Table 1. Alternative activities for knights.</figcaption>
     <table>
      <tr>
       <th> Activity
       <th> Location
       <th> Cost
      <tr>
       <td> Dance
       <td> Wherever possible
       <td> £0<sup><a href="#fn1">1</a></sup>
      <tr>
       <td> Routines, chorus scenes<sup><a href="#fn2">2</a></sup>
       <td> Undisclosed
       <td> Undisclosed
      <tr>
       <td> Dining<sup><a href="#fn3">3</a></sup>
       <td> Camelot
       <td> Cost of ham, jam, and spam<sup><a href="#fn4">4</a></sup>
     </table>
     <p id="fn1">1. Assumed.</p>
     <p id="fn2">2. Footwork impeccable.</p>
     <p id="fn3">3. Quality described as "well".</p>
     <p id="fn4">4. A lot.</p>
    </figure>

### 4.14 Disabled elements

An element is said to be actually disabled if it one of the following:

- a `button` element that is disabled
- an `input` element that is disabled
- a `select` element that is disabled
- a `textarea` element that is disabled
- an `optgroup` element that has a `disabled` attribute
- an `option` element that is disabled
- a `menuitem` element that has a `disabled` attribute
- a `fieldset` element that is a disabled fieldset

This definition is used to determine what elements can be focused and which elements match the `:disabled` pseudo-class.

### 4.15 Matching HTML elements using selectors

#### 4.15.1 Case-sensitivity

The Selectors specification leaves the case-sensitivity of element names, attribute names, and attribute values to be defined by the host language. [SELECTORS]

The Selectors specification defines that ID and class selectors, when matched against elements in documents that are in quirks mode, will be work in an ASCII case-insensitive.

When comparing a CSS element type selector to the names of HTML elements in HTML documents, the CSS element type selector must first be converted to ASCII lowercase. The same selector when compared to other elements must be compared according to its original case. In both cases, the comparison is case-sensitive.

When comparing the name part of a CSS attribute selector to the names of namespace-less attributes on HTML elements in HTML documents, the name part of the CSS attribute selector must first be converted to ASCII lowercase. The same selector when compared to other attributes must be compared according to its original case. In both cases, the comparison is case-sensitive.

Everything else must be treated as entirely case-sensitive for the purposes of selector matching. This includes:

- IDs and classes in no-quirks mode and limited-quirks mode
- the names of elements not in the HTML namespace
- the names of HTML elements in XML documents
- the names of attributes of elements not in the HTML namespace
- the names of attributes of HTML elements in XML documents
- the names of attributes that themselves have namespaces
- all attribute values

#### 4.15.2 Pseudo-classes

There are a number of dynamic selectors that can be used with HTML. This section defines when these selectors match HTML elements. [SELECTORS] [CSSUI]

`:link`
`:visited`

  All `a` elements that have an `href` attribute, all `area` elements that have an `href` attribute, and all `link` elements that have an `href` attribute, must match one of `:link` and `:visited`.

  Other specifications might apply more specific rules regarding how these elements are to match these pseudo-classes, to mitigate some privacy concerns that apply with straightforward implementations of this requirement.

`:active`

  The `:active` pseudo-class is defined to match an element while an element is being activated by the user.

  To determine whether a particular element is being activated for the purposes of defining the `:active` pseudo-class only, an HTML user agent must use the first relevant entry in the following list.

  If the element has a descendant that is currently matching the `:active` pseudo-class

    The element is being activated.

  If the element is the labeled control of a `label` element that is currently matching :active

    The element is being activated.

  If the element is a `button` element
  If the element is an `input` element whose `type` attribute is in the Submit Button, Image Button, Reset Button, or Button state

    The element is being activated if it is in a formal activation state and it is not disabled.

    For example, if the user is using a keyboard to push a `button` element by pressing the space bar, the element would match this pseudo-class in between the time that the element received the `keydown` event and the time the element received the `keyup` event.

  If the element is a `menuitem` element

    The element is being activated if it is in a formal activation state and it does not have a `disabled` attribute.

  If the element is an `a` element that has an `href` attribute
  If the element is an `area` element that has an `href` attribute
  If the element is a `link` element that has an `href` attribute
  If the element has its tabindex focus flag set

    The element is being activated if it is in a formal activation state.

  If the element is being actively pointed at

    The element is being activated.

  An element is said to be in a formal activation state between the time the user begins to indicate an intent to trigger the element's activation behaviour and either the time the user stops indicating an intent to trigger the element's activation behaviour, or the time the element's activation behaviour has finished running, which ever comes first.

  An element is said to be being actively pointed at while the user indicates the element using a pointing device while that pointing device is in the "down" state (e.g. for a mouse, between the time the mouse button is pressed and the time it is depressed; for a finger in a multitouch environment, while the finger is touching the display surface).

`:hover`

  The `:hover` pseudo-class is defined to match an element while the user designates an element with a pointing device. For the purposes of defining the `:hover` pseudo-class only, an HTML user agent must consider an element as being one that the user designates if it is:

  - An element that the user indicates using a pointing device.

  - An element that has a descendant that the user indicates using a pointing device.

  - An element that is the labeled control of a `label` element that is currently matching :hover.

  Consider in particular a fragment such as:

      <p> <label for=c> <input id=a> </label> <span id=b> <input id=c> </span> </p>

  If the user designates the element with ID "`a`" with their pointing device, then the `p` element (and all its ancestors not shown in the snippet above), the `label` element, the element with ID "`a`", and the element with ID "`c`" will match the :hover pseudo-class. The element with ID "`a`" matches it from condition 1, the `label` and `p` elements match it because of condition 2 (one of their descendants is designated), and the element with ID "`c`" matches it through condition 3 (its `label` element matches :hover). However, the element with ID "`b`" does not match :hover: its descendant is not designated, even though it matches :hover.

`:focus`

  For the purposes of the CSS ':focus' pseudo-class, an element has the focus when its top-level browsing context has the system focus, it is not itself a browsing context container, and it is one of the elements listed in the focus chain of the currently focused area of the top-level browsing context.

`:enabled`

  The `:enabled` pseudo-class must match any element that is one of the following:

  - a `button` element that is not disabled
  - an `input` element that is not disabled
  - a `select` element that is not disabled
  - a `textarea` element that is not disabled
  - an `optgroup` element that does not have a `disabled` attribute
  - an `option` element that is not disabled
  - a `menuitem` element that does not have a `disabled` attribute
  - a `fieldset` element that is not a disabled fieldset
`:disabled`

  The `:disabled` pseudo-class must match any element that is actually disabled.

`:checked`

  The `:checked` pseudo-class must match any element falling into one of the following categories:

  - `input` elements whose `type` attribute is in the Checkbox state and whose checkedness state is true
  - `input` elements whose `type` attribute is in the Radio Button state and whose checkedness state is true
  - `option` elements whose selectedness is true
  - `menuitem` elements whose `type` attribute is in the Checkbox state and that have a `checked` attribute
  - `menuitem` elements whose `type` attribute is in the Radio state and that have a `checked` attribute
`:indeterminate`

  The `:indeterminate` pseudo-class must match any element falling into one of the following categories:

  - `input` elements whose `type` attribute is in the Checkbox state and whose `indeterminate` IDL attribute is set to true
  - `input` elements whose `type` attribute is in the Radio Button state and whose radio button group contains no `input` elements whose checkedness state is true.
  - `progress` elements with no `value` content attribute
`:default`

  The `:default` pseudo-class must match any element falling into one of the following categories:

  - `button` elements that are their form's default button
  - `input` elements whose `type` attribute is in the Submit Button or Image Button state, and that are their form's default button
  - `input` elements to which the `checked` attribute applies and that have a `checked` attribute
  - `option` elements that have a `selected` attribute
`:valid`

  The `:valid` pseudo-class must match any element falling into one of the following categories:

  - elements that are candidates for constraint validation and that satisfy their constraints
  - `form` elements that are not the form owner of any elements that themselves are candidates for constraint validation but do not satisfy their constraints
  - `fieldset` elements that have no descendant elements that themselves are candidates for constraint validation but do not satisfy their constraints
`:invalid`

  The `:invalid` pseudo-class must match any element falling into one of the following categories:

  - elements that are candidates for constraint validation but that do not satisfy their constraints
  - `form` elements that are the form owner of one or more elements that themselves are candidates for constraint validation but do not satisfy their constraints
  - `fieldset` elements that have of one or more descendant elements that themselves are candidates for constraint validation but do not satisfy their constraints
`:in-range`

  The `:in-range` pseudo-class must match all elements that are candidates for constraint validation, have range limitations, and that are neither suffering from an underflow nor suffering from an overflow.

`:out-of-range`

  The `:out-of-range` pseudo-class must match all elements that are candidates for constraint validation, have range limitations, and that are either suffering from an underflow or suffering from an overflow.

`:required`

  The `:required` pseudo-class must match any element falling into one of the following categories:

  - `input` elements that are required
  - `select` elements that have a `required` attribute
  - `textarea` elements that have a `required` attribute
`:optional`

  The `:optional` pseudo-class must match any element falling into one of the following categories:

  - `input` elements to which the `required` attribute applies that are not required
  - `select` elements that do not have a `required` attribute
  - `textarea` elements that do not have a `required` attribute
`:read-only`
`:read-write`

  The `:read-write` pseudo-class must match any element falling into one of the following categories, which for the purposes of Selectors are thus considered user-alterable: [SELECTORS]

  - `input` elements to which the `readonly` attribute applies, and that are mutable (i.e. that do not have the `readonly` attribute specified and that are not disabled)
  - `textarea` elements that do not have a `readonly` attribute, and that are not disabled
  - elements that are editing hosts or editable and are neither `input` elements nor `textarea` elements

  The `:read-only` pseudo-class must match all other HTML elements.

`:dir(ltr)`

  The `:dir(ltr)` pseudo-class must match all elements whose directionality is 'ltr'.

`:dir(rtl)`

  The `:dir(rtl)` pseudo-class must match all elements whose directionality is 'rtl'.

Another section of this specification defines the target element used with the `:target` pseudo-class.

This specification does not define when an element matches the `:lang()` dynamic pseudo-class, as it is defined in sufficient detail in a language-agnostic fashion in the Selectors specification. [SELECTORS]