## 2 Common infrastructure

### 2.1 Terminology

This specification refers to both HTML and XML attributes and IDL attributes, often in the same context. When it is not clear which is being referred to, they are referred to as content attributes for HTML and XML attributes, and IDL attributes for those defined on IDL interfaces. Similarly, the term "properties" is used for both JavaScript object properties and CSS properties. When these are ambiguous they are qualified as object properties and CSS properties respectively.

Generally, when the specification states that a feature applies to the HTML syntax or the XHTML syntax, it also includes the other. When a feature specifically only applies to one of the two languages, it is called out by explicitly stating that it does not apply to the other format, as in "for HTML, ... (this does not apply to XHTML)".

This specification uses the term document to refer to any use of HTML, ranging from short static documents to long essays or reports with rich multimedia, as well as to fully-fledged interactive applications. The term is used to refer both to `Document` objects and their descendant DOM trees, and to serialised byte streams using the HTML syntax or XHTML syntax, depending on context.

In the context of the DOM structures, the terms HTML document and XML document are used as defined in the DOM specification, and refer specifically to two different modes that `Document` objects can find themselves in. [DOM] (Such uses are always hyperlinked to their definition.)

In the context of byte streams, the term HTML document refers to resources labeled as `text/html`, and the term XML document refers to resources labeled with an XML MIME type.

The term XHTML document is used to refer to both `Document`s in the XML document mode that contains element nodes in the HTML namespace, and byte streams labeled with an XML MIME type that contain elements from the HTML namespace, depending on context.

For simplicity, terms such as shown, displayed, and visible might sometimes be used when referring to the way a document is rendered to the user. These terms are not meant to imply a visual medium; they must be considered to apply to other media in equivalent ways.

When an algorithm B says to return to another algorithm A, it implies that A called B. Upon returning to A, the implementation must continue from where it left off in calling B. Some algorithms run in parallel; this means that the algorithm's subsequent steps are to be run, one after another, at the same time as other logic in the specification (e.g. at the same time as the event loop). This specification does not define the precise mechanism by which this is achieved, be it time-sharing cooperative multitasking, fibers, threads, processes, using different hyperthreads, cores, CPUs, machines, etc. By contrast, an operation that is to run immediately must interrupt the currently running task, run itself, and then resume the previously running task.

The term "transparent black" refers to the colour with red, green, blue, and alpha channels all set to zero.

#### 2.1.1 Resources

The specification uses the term supported when referring to whether a user agent has an implementation capable of decoding the semantics of an external resource. A format or type is said to be supported if the implementation can process an external resource of that format or type without critical aspects of the resource being ignored. Whether a specific resource is supported can depend on what features of the resource's format are in use.

For example, a PNG image would be considered to be in a supported format if its pixel data could be decoded and rendered, even if, unbeknownst to the implementation, the image also contained animation data.

An MPEG-4 video file would not be considered to be in a supported format if the compression format used was not supported, even if the implementation could determine the dimensions of the movie from the file's metadata.

What some specifications, in particular the HTTP specification, refer to as a representation is referred to in this specification as a resource. [HTTP]

The term MIME type is used to refer to what is sometimes called an Internet media type in protocol literature. The term media type in this specification is used to refer to the type of media intended for presentation, as used by the CSS specifications. [RFC2046] [MQ]

A string is a valid MIME type if it matches the `media-type` rule defined in section 3.1.1.1 "Media Type" of RFC 7231. In particular, a valid MIME type may include MIME type parameters. [HTTP]

A string is a valid MIME type with no parameters if it matches the `media-type` rule defined in section 3.1.1.1 "Media Type" of RFC 7231, but does not contain any U+003B SEMICOLON characters (;). In other words, if it consists only of a type and subtype, with no MIME Type parameters. [HTTP]

The term HTML MIME type is used to refer to the MIME type `text/html`.

A resource's critical subresources are those that the resource needs to have available to be correctly processed. Which resources are considered critical or not is defined by the specification that defines the resource's format.

#### 2.1.2 XML

To ease migration from HTML to XHTML, UAs conforming to this specification will place elements in HTML in the `http://www.w3.org/1999/xhtml` namespace, at least for the purposes of the DOM and CSS. The term "HTML elements", when used in this specification, refers to any element in that namespace, and thus refers to both HTML and XHTML elements.

Except where otherwise stated, all elements defined or mentioned in this specification are in the HTML namespace ("`http://www.w3.org/1999/xhtml`"), and all attributes defined or mentioned in this specification have no namespace.

The term element type is used to refer to the set of elements that have a given local name and namespace. For example, `button` elements are elements with the element type `button`, meaning they have the local name "`button`" and (implicitly as defined above) the HTML namespace.

Attribute names are said to be XML-compatible if they match the `Name` production defined in XML and they contain no U+003A COLON characters (:). [XML]

The term XML MIME type is used to refer to the MIME types `text/xml`, `application/xml`, and any MIME type whose subtype ends with the four characters "`+xml`". [RFC7303]

#### 2.1.3 DOM trees

The root element of a `Document` object is that `Document`'s first element child, if any. If it does not have one then the `Document` has no root element.

The term root element, when not referring to a `Document` object's root element, means the furthest ancestor element node of whatever node is being discussed, or the node itself if it has no ancestors. When the node is a part of the document, then the node's root element is indeed the document's root element; however, if the node is not currently part of the document tree, the root element will be an orphaned node.

When an element's root element is the root element of a `Document` object, it is said to be in a `Document`. An element is said to have been inserted into a document when its root element changes and is now the document's root element. Analogously, an element is said to have been removed from a document when its root element changes from being the document's root element to being another element.

A node's home subtree is the subtree rooted at that node's root element. When a node is in a `Document`, its home subtree is that `Document`'s tree.

The term tree order means a pre-order, depth-first traversal of DOM nodes involved (through the `parentNode`/`childNodes` relationship).

When it is stated that some element or attribute is ignored, or treated as some other value, or handled as if it was something else, this refers only to the processing of the node after it is in the DOM. A user agent must not mutate the DOM in such situations.

A content attribute is said to change value only if its new value is different than its previous value; setting an attribute to a value it already has does not change it.

The term empty, when used of an attribute value, `Text` node, or string, means that the length of the text is zero (i.e. not even containing spaces or control characters).

A node A is inserted into a node B when the insertion steps are invoked with A as the argument and A's new parent is B. Similarly, a node A is removed from a node B when the removing steps are invoked with A as the removedNode argument and B as the oldParent argument.

#### 2.1.4 Scripting

The construction "a `Foo` object", where `Foo` is actually an interface, is sometimes used instead of the more accurate "an object implementing the interface `Foo`".

An IDL attribute is said to be getting when its value is being retrieved (e.g. by author script), and is said to be setting when a new value is assigned to it.

If a DOM object is said to be live, then the attributes and methods on that object must operate on the actual underlying data, not a snapshot of the data.

In the contexts of events, the terms fire and dispatch are used as defined in the DOM specification: firing an event means to create and dispatch it, and dispatching an event means to follow the steps that propagate the event through the tree. The term trusted event is used to refer to events whose `isTrusted` attribute is initialised to true. [DOM]

#### 2.1.5 Plugins

The term plugin refers to a user-agent defined set of content handlers used by the user agent that can take part in the user agent's rendering of a `Document` object, but that neither act as child browsing contexts of the `Document` nor introduce any `Node` objects to the `Document`'s DOM.

Typically such content handlers are provided by third parties, though a user agent can also designate built-in content handlers as plugins.

A user agent must not consider the types `text/plain` and `application/octet-stream` as having a registered plugin.

One example of a plugin would be a PDF viewer that is instantiated in a browsing context when the user navigates to a PDF file. This would count as a plugin regardless of whether the party that implemented the PDF viewer component was the same as that which implemented the user agent itself. However, a PDF viewer application that launches separate from the user agent (as opposed to using the same interface) is not a plugin by this definition.

This specification does not define a mechanism for interacting with plugins, as it is expected to be user-agent- and platform-specific. Some UAs might opt to support a plugin mechanism such as the Netscape Plugin API; others might use remote content converters or have built-in support for certain types. Indeed, this specification doesn't require user agents to support plugins at all. [NPAPI]

A plugin can be secured if it honors the semantics of the `sandbox` attribute.

For example, a secured plugin would prevent its contents from creating pop-up windows when the plugin is instantiated inside a sandboxed `iframe`.

Browsers should take extreme care when interacting with external content intended for plugins. When third-party software is run with the same privileges as the user agent itself, vulnerabilities in the third-party software become as dangerous as those in the user agent.

Since different users having differents sets of plugins provides a fingerprinting vector that increases the chances of users being uniquely identified, user agents are encouraged to support the exact same set of plugins for each user.

#### 2.1.6 Character encodings

A character encoding, or just encoding where that is not ambiguous, is a defined way to convert between byte streams and Unicode strings, as defined in the WHATWG Encoding standard. An encoding has an encoding name and one or more encoding labels, referred to as the encoding's name and labels in the Encoding standard. [ENCODING]

An ASCII-compatible character encoding is a single-byte or variable-length encoding in which the bytes 0x09, 0x0A, 0x0C, 0x0D, 0x20 - 0x22, 0x26, 0x27, 0x2C - 0x3F, 0x41 - 0x5A, and 0x61 - 0x7A, ignoring bytes that are the second and later bytes of multibyte sequences, all correspond to single-byte sequences that map to the same Unicode characters as those bytes in Windows-1252. [ENCODING]

This includes such encodings as Shift_JIS, HZ-GB-2312, and variants of ISO-2022, even though it is possible in these encodings for bytes like 0x70 to be part of longer sequences that are unrelated to their interpretation as ASCII. It excludes UTF-16 variants, as well as obsolete legacy encodings such as UTF-7, GSM03.38, and EBCDIC variants.

The term a UTF-16 encoding refers to any variant of UTF-16: UTF-16LE or UTF-16BE, regardless of the presence or absence of a BOM. [ENCODING]

The term code unit is used as defined in the Web IDL specification: a 16 bit unsigned integer, the smallest atomic component of a `DOMString`. (This is a narrower definition than the one used in Unicode, and is not the same as a code point.) [WEBIDL]

The term Unicode code point means a Unicode scalar value where possible, and an isolated surrogate code point when not. When a conformance requirement is defined in terms of characters or Unicode code points, a pair of code units consisting of a high surrogate followed by a low surrogate must be treated as the single code point represented by the surrogate pair, but isolated surrogates must each be treated as the single code point with the value of the surrogate. [UNICODE]

In this specification, the term character, when not qualified as Unicode character, is synonymous with the term Unicode code point.

The term Unicode character is used to mean a Unicode scalar value (i.e. any Unicode code point that is not a surrogate code point). [UNICODE]

The code-unit length of a string is the number of code units in that string.

This complexity results from the historical decision to define the DOM API in terms of 16 bit (UTF-16) code units, rather than in terms of Unicode characters.

### 2.2 Conformance requirements

All diagrams, examples, and notes in this specification are non-normative, as are all sections explicitly marked non-normative. Everything else in this specification is normative.

The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC2119. The key word "OPTIONALLY" in the normative parts of this document is to be interpreted with the same normative meaning as "MAY" and "OPTIONAL". For readability, these words do not appear in all uppercase letters in this specification. [RFC2119]

Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort these steps") are to be interpreted with the meaning of the key word ("must", "should", "may", etc) used in introducing the algorithm.

For example, were the spec to say:

    To eat an orange, the user must:
    1. Peel the orange.
    2. Separate each slice of the orange.
    3. Eat the orange slices.

...it would be equivalent to the following:

    To eat an orange:
    1. The user must peel the orange.
    2. The user must separate each slice of the orange.
    3. The user must eat the orange slices.

Here the key word is "must".

The former (imperative) style is generally preferred in this specification for stylistic reasons.

Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the end result is equivalent. (In particular, the algorithms defined in this specification are intended to be easy to follow, and not intended to be performant.)

#### 2.2.1 Conformance classes

This specification describes the conformance criteria for user agents (relevant to implementors) and documents (relevant to authors and authoring tool implementors).

Conforming documents are those that comply with all the conformance criteria for documents. For readability, some of these conformance requirements are phrased as conformance requirements on authors; such requirements are implicitly requirements on documents: by definition, all documents are assumed to have had an author. (In some cases, that author may itself be a user agent — such user agents are subject to additional rules, as explained below.)

For example, if a requirement states that "authors must not use the `foobar` element", it would imply that documents are not allowed to contain elements named `foobar`.

There is no implied relationship between document conformance requirements and implementation conformance requirements. User agents are not free to handle non-conformant documents as they please; the processing model described in this specification applies to implementations regardless of the conformity of the input documents.

User agents fall into several (overlapping) categories with different conformance requirements.

Web browsers and other interactive user agents

  Web browsers that support the XHTML syntax must process elements and attributes from the HTML namespace found in XML documents as described in this specification, so that users can interact with them, unless the semantics of those elements have been overridden by other specifications.

  A conforming XHTML processor would, upon finding an XHTML `script` element in an XML document, execute the script contained in that element. However, if the element is found within a transformation expressed in XSLT (assuming the user agent also supports XSLT), then the processor would instead treat the `script` element as an opaque element that forms part of the transform.

  Web browsers that support the HTML syntax must process documents labeled with an HTML MIME type as described in this specification, so that users can interact with them.

  User agents that support scripting must also be conforming implementations of the IDL fragments in this specification, as described in the Web IDL specification. [WEBIDL]

  Unless explicitly stated, specifications that override the semantics of HTML elements do not override the requirements on DOM objects representing those elements. For example, the `script` element in the example above would still implement the `HTMLScriptElement` interface.

Non-interactive presentation user agents

  User agents that process HTML and XHTML documents purely to render non-interactive versions of them must comply to the same conformance criteria as Web browsers, except that they are exempt from requirements regarding user interaction.

  Typical examples of non-interactive presentation user agents are printers (static UAs) and overhead displays (dynamic UAs). It is expected that most static non-interactive presentation user agents will also opt to lack scripting support.

  A non-interactive but dynamic presentation UA would still execute scripts, allowing forms to be dynamically submitted, and so forth. However, since the concept of "focus" is irrelevant when the user cannot interact with the document, the UA would not need to support any of the focus-related DOM APIs.

Visual user agents that support the suggested default rendering

  User agents, whether interactive or not, may be designated (possibly as a user option) as supporting the suggested default rendering defined by this specification.

  This is not required. In particular, even user agents that do implement the suggested default rendering are encouraged to offer settings that override this default to improve the experience for the user, e.g. changing the colour contrast, using different focus styles, or otherwise making the experience more accessible and usable to the user.

  User agents that are designated as supporting the suggested default rendering must, while so designated, implement the rules in the rendering section that that section defines as the behaviour that user agents are expected to implement.

User agents with no scripting support

  Implementations that do not support scripting (or which have their scripting features disabled entirely) are exempt from supporting the events and DOM interfaces mentioned in this specification. For the parts of this specification that are defined in terms of an events model or in terms of the DOM, such user agents must still act as if events and the DOM were supported.

  Scripting can form an integral part of an application. Web browsers that do not support scripting, or that have scripting disabled, might be unable to fully convey the author's intent.

Conformance checkers

  Conformance checkers must verify that a document conforms to the applicable conformance criteria described in this specification. Automated conformance checkers are exempt from detecting errors that require interpretation of the author's intent (for example, while a document is non-conforming if the content of a `blockquote` element is not a quote, conformance checkers running without the input of human judgement do not have to check that `blockquote` elements only contain quoted material).

  Conformance checkers must check that the input document conforms when parsed without a browsing context (meaning that no scripts are run, and that the parser's scripting flag is disabled), and should also check that the input document conforms when parsed with a browsing context in which scripts execute, and that the scripts never cause non-conforming states to occur other than transiently during script execution itself. (This is only a "SHOULD" and not a "MUST" requirement because it has been proven to be impossible. [COMPUTABLE])

  The term "HTML validator" can be used to refer to a conformance checker that itself conforms to the applicable requirements of this specification.

  XML DTDs cannot express all the conformance requirements of this specification. Therefore, a validating XML processor and a DTD cannot constitute a conformance checker. Also, since neither of the two authoring formats defined in this specification are applications of SGML, a validating SGML system cannot constitute a conformance checker either.

  To put it another way, there are three types of conformance criteria:

  1. Criteria that can be expressed in a DTD.
  2. Criteria that cannot be expressed by a DTD, but can still be checked by a machine.
  3. Criteria that can only be checked by a human.

  A conformance checker must check for the first two. A simple DTD-based validator only checks for the first class of errors and is therefore not a conforming conformance checker according to this specification.

Data mining tools

  Applications and tools that process HTML and XHTML documents for reasons other than to either render the documents or check them for conformance should act in accordance with the semantics of the documents that they process.

  A tool that generates document outlines but increases the nesting level for each paragraph and does not increase the nesting level for each section would not be conforming.

Authoring tools and markup generators

  Authoring tools and markup generators must generate conforming documents. Conformance criteria that apply to authors also apply to authoring tools, where appropriate.

  Authoring tools are exempt from the strict requirements of using elements only for their specified purpose, but only to the extent that authoring tools are not yet able to determine author intent. However, authoring tools must not automatically misuse elements or encourage their users to do so.

  For example, it is not conforming to use an `address` element for arbitrary contact information; that element can only be used for marking up contact information for the author of the document or section. However, since an authoring tool is likely unable to determine the difference, an authoring tool is exempt from that requirement. This does not mean, though, that authoring tools can use `address` elements for any block of italics text (for instance); it just means that the authoring tool doesn't have to verify that when the user uses a tool for inserting contact information for a section, that the user really is doing that and not inserting something else instead.

  In terms of conformance checking, an editor has to output documents that conform to the same extent that a conformance checker will verify.

  When an authoring tool is used to edit a non-conforming document, it may preserve the conformance errors in sections of the document that were not edited during the editing session (i.e. an editing tool is allowed to round-trip erroneous content). However, an authoring tool must not claim that the output is conformant if errors have been so preserved.

  Authoring tools are expected to come in two broad varieties: tools that work from structure or semantic data, and tools that work on a What-You-See-Is-What-You-Get media-specific editing basis (WYSIWYG).

  The former is the preferred mechanism for tools that author HTML, since the structure in the source information can be used to make informed choices regarding which HTML elements and attributes are most appropriate.

  However, WYSIWYG tools are legitimate. WYSIWYG tools should use elements they know are appropriate, and should not use elements that they do not know to be appropriate. This might in certain extreme cases mean limiting the use of flow elements to just a few elements, like `div`, `b`, `i`, and `span` and making liberal use of the `style` attribute.

  All authoring tools, whether WYSIWYG or not, should make a best effort attempt at enabling users to create well-structured, semantically rich, media-independent content.

User agents may impose implementation-specific limits on otherwise unconstrained inputs, e.g. to prevent denial of service attacks, to guard against running out of memory, or to work around platform-specific limitations.

For compatibility with existing content and prior specifications, this specification describes two authoring formats: one based on XML (referred to as the XHTML syntax), and one using a custom format inspired by SGML (referred to as the HTML syntax). Implementations must support at least one of these two formats, although supporting both is encouraged.

Some conformance requirements are phrased as requirements on elements, attributes, methods or objects. Such requirements fall into two categories: those describing content model restrictions, and those describing implementation behaviour. Those in the former category are requirements on documents and authoring tools. Those in the second category are requirements on user agents. Similarly, some conformance requirements are phrased as requirements on authors; such requirements are to be interpreted as conformance requirements on the documents that authors produce. (In other words, this specification does not distinguish between conformance criteria on authors and conformance criteria on documents.)

#### 2.2.2 Dependencies

This specification relies on several other underlying specifications.

Unicode and Encoding

  The Unicode character set is used to represent textual data, and the WHATWG Encoding standard defines requirements around character encodings. [UNICODE]

  This specification introduces terminology based on the terms defined in those specifications, as described earlier.

  The following terms are used as defined in the WHATWG Encoding standard: [ENCODING]

  - Getting an encoding
  - The encoder and decoder algorithms for various encodings, including the UTF-8 encoder and UTF-8 decoder
  - The generic decode algorithm which takes a byte stream and an encoding and returns a character stream
  - The UTF-8 decode algorithm which takes a byte stream and returns a character stream, additionally stripping one leading UTF-8 Byte Order Mark (BOM), if any

  The UTF-8 decoder is distinct from the UTF-8 decode algorithm. The latter first strips a Byte Order Mark (BOM), if any, and then invokes the former.

  For readability, character encodings are sometimes referenced in this specification with a case that differs from the canonical case given in the WHATWG Encoding standard. (For example, "UTF-16LE" instead of "utf-16le".)

XML and related specifications

  Implementations that support the XHTML syntax must support some version of XML, as well as its corresponding namespaces specification, because that syntax uses an XML serialisation with namespaces. [XML] [XMLNS]

  The attribute with the tag name `xml:space` in the XML namespace is defined by the XML specification. [XML]

  This specification also references the `<?xml-stylesheet?>` processing instruction, defined in the Associating Style Sheets with XML documents specification. [XMLSSPI]

  This specification also non-normatively mentions the `XSLTProcessor` interface and its `transformToFragment()` and `transformToDocument()` methods.

URLs

  The following terms are defined in the WHATWG URL standard: [URL]

  - URL
  - Origin of URLs
  - Absolute URL
  - Relative URL
  - Relative schemes
  - The URL parser
  - Parsed URL
  - The scheme component of a parsed URL
  - The scheme data component of a parsed URL
  - The username component of a parsed URL
  - The password component of a parsed URL
  - The host component of a parsed URL
  - The port component of a parsed URL
  - The path component of a parsed URL
  - The query component of a parsed URL
  - The fragment component of a parsed URL
  - Parse errors from the URL parser
  - The URL serialiser
  - Default encode set
  - Percent encode
  - UTF-8 percent encode
  - Percent decode
  - Decoder error
  - The domain to ASCII algorithm
  - The domain to Unicode algorithm
  - `URLUtils` interface
  - `URLUtilsReadOnly` interface
  - `href` IDL attribute
  - `protocol` IDL attribute
  - The get the base hook for `URLUtils`
  - The update steps hook for `URLUtils`
  - The set the input algorithm for `URLUtils`
  - The query encoding of an `URLUtils` object
  - The input of an `URLUtils` object
  - The url of an `URLUtils` object

  A number of schemes and protocols are referenced by this specification also:

  - The `about:` scheme [ABOUT]
  - The `blob:` scheme [FILEAPI]
  - The `data:` scheme [RFC2397]
  - The `http:` scheme [HTTP]
  - The `https:` scheme [HTTP]
  - The `mailto:` scheme [MAILTO]
  - The `sms:` scheme [SMS]
  - The `urn:` scheme [URN]
HTTP and related specifications

  The following terms are defined in the HTTP specifications: [HTTP]

  - `Accept` header
  - `Accept-Language` header
  - `Cache-Control` header
  - `Content-Disposition` header
  - `Content-Language` header
  - `Content-Length` header
  - `Host` header
  - `Last-Modified` header
  - `Referer` header

  The following term is defined in the Web Origin specification: [ORIGIN]

  - `Origin` header

  The following terms are defined in the Cookie specification: [COOKIES]

  - cookie-string
  - receives a set-cookie-string
  - `Cookie` header

  The following term is defined in the Web Linking specification: [WEBLINK]

  - `Link` header
Fetch

  The following terms are defined in the WHATWG Fetch specification: [FETCH]

  - cross-origin request
  - cross-origin request status
  - custom request headers
  - simple cross-origin request
  - redirect steps
  - omit credentials flag
  - resource sharing check

  This specification does not yet use the "fetch" algorithm from the WHATWG Fetch specification. It will be updated to do so in due course. Some of the terms mentioned above are obsolete; this will be fixed when the spec is updated to use the new algorithm.

Web IDL

  The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in the Web IDL specification. [WEBIDL]

  The following terms are defined in the Web IDL specification:

  - Supported property indices
  - Determine the value of an indexed property
  - Support named properties
  - Supported property names
  - Unenumerable
  - Determine the value of a named property
  - Platform array objects
  - Read only (when applied to arrays)
  - Callback this value
  - Converting between WebIDL types and JS types
  - invoke the Web IDL callback function

  The Web IDL specification also defines the following types that are used in Web IDL fragments in this specification:

  - `Array`
  - `boolean`
  - `Boolean`
  - `Date`
  - `DOMString`
  - `USVString`
  - `double`
  - `Error`
  - `Function`
  - `long`
  - `Map`
  - `Number`
  - `Object`
  - `RegExp`
  - `Set`
  - `String`
  - `unrestricted double`
  - `unsigned long`

  The term throw in this specification is used as defined in the WebIDL specification. The following exception names are defined by WebIDL and used by this specification:

  1. `IndexSizeError`
  2. `HierarchyRequestError`
  3. `InvalidCharacterError`
  4. `NotFoundError`
  5. `NotSupportedError`
  6. `InvalidStateError`
  7. `SyntaxError`
  8. `InvalidAccessError`
  9. `SecurityError`
  10. `NetworkError`
  11. `URLMismatchError`
  12. `QuotaExceededError`
  13. `TimeoutError`
  14. `DataCloneError`
  15. `TypeError`

  When this specification requires a user agent to create a `Date` object representing a particular time (which could be the special value Not-a-Number), the milliseconds component of that time, if any, must be truncated to an integer, and the time value of the newly created `Date` object must represent the resulting truncated time.

  For instance, given the time 23045 millionths of a second after 01:00 UTC on January 1st 2000, i.e. the time 2000-01-01T00:00:00.023045Z, then the `Date` object created representing that time would represent the same time as that created representing the time 2000-01-01T00:00:00.023Z, 45 millionths earlier. If the given time is NaN, then the result is a `Date` object that represents a time value NaN (indicating that the object does not represent a specific instant of time).

JavaScript

  Some parts of the language described by this specification only support JavaScript as the underlying scripting language. [ECMA262]

  The term "JavaScript" is used to refer to ECMA262, rather than the official term ECMAScript, since the term JavaScript is more widely known. Similarly, the MIME type used to refer to JavaScript in this specification is `text/javascript`, since that is the most commonly used type, despite it being an officially obsoleted type according to RFC 4329. [RFC4329]

  The term JavaScript global environment refers to the global environment concept defined in the ECMAScript specification.

  The ECMAScript `SyntaxError` exception is also defined in the ECMAScript specification. [ECMA262]

  The ArrayBuffer and related object types and underlying concepts from the ECMAScript Specification are used for several features in this specification. [ECMA262]

  The following helper IDL is used for referring to `ArrayBuffer`-related types:

      typedef (Int8Array or Uint8Array or Uint8ClampedArray or
               Int16Array or Uint16Array or
               Int32Array or Uint32Array or
               Float32Array or Float64Array or
               DataView) ArrayBufferView;

  In particular, the `Uint8ClampedArray` type is used by some 2D canvas APIs, and the `WebSocket` API uses `ArrayBuffer` objects for handling binary frames.

DOM

  The Document Object Model (DOM) is a representation — a model — of a document and its content. The DOM is not just an API; the conformance criteria of HTML implementations are defined, in this specification, in terms of operations on the DOM. [DOM]

  Implementations must support DOM and the events defined in DOM Events, because this specification is defined in terms of the DOM, and some of the features are defined as extensions to the DOM interfaces. [DOM] [DOMEVENTS]

  In particular, the following features are defined in the DOM specification: [DOM]

  - `Attr` interface
  - `Comment` interface
  - `DOMImplementation` interface
  - `Document` interface
  - `XMLDocument` interface
  - `DocumentFragment` interface
  - `DocumentType` interface
  - `ChildNode` interface
  - `Element` interface
  - `Node` interface
  - `NodeList` interface
  - `ProcessingInstruction` interface
  - `Text` interface
  - node document concept
  - `HTMLCollection` interface
  - `HTMLCollection.length` attribute
  - `HTMLCollection.item()` method
  - `HTMLCollection.namedItem()` method
  - The terms collections and represented by the collection
  - `DOMTokenList` interface
  - `DOMSettableTokenList` interface
  - `DOMSettableTokenList.value` attribute
  - `createDocument()` method
  - `createHTMLDocument()` method
  - `createElement()` method
  - `createElementNS()` method
  - `getElementById()` method
  - `getElementsByClassName()` method
  - `insertBefore()` method
  - `appendChild()` method
  - `cloneNode()` method
  - `importNode()` method
  - `childNodes` attribute
  - `localName` attribute
  - `parentNode` attribute
  - `namespaceURI` attribute
  - `tagName` attribute
  - `id` attribute
  - `textContent` attribute
  - The insert, append, remove, replace, and adopt algorithms for nodes
  - The insertion steps, removing steps, and adopting steps hooks
  - The attribute list concept.
  - The data of a text node.
  - `Event` interface
  - `EventTarget` interface
  - `EventInit` dictionary type
  - `target` attribute
  - `currentTarget` attribute
  - `isTrusted` attribute
  - `initEvent()` method
  - `addEventListener()` method
  - The type of an event
  - The concept of an event listener and the event listeners associated with an `EventTarget`
  - The concept of a target override
  - The concept of a regular event parent and a cross-boundary event parent
  - The encoding (herein the character encoding) and content type of a `Document`
  - The distinction between XML documents and HTML documents
  - The terms quirks mode, limited-quirks mode, and no-quirks mode
  - The algorithm to clone a `Node`, and the concept of cloning steps used by that algorithm
  - The concept of base URL change steps and the definition of what happens when an element is affected by a base URL change
  - The concept of an element's unique identifier (ID)
  - The concept of a DOM range, and the terms start, end, and boundary point as applied to ranges.
  - `MutationObserver` interface and mutation observers in general
  - Promise interface
  - The resolver concept
  - The fulfill and reject algorithms

  The following features are defined in the DOM Events specification: [DOMEVENTS]

  - `MouseEvent` interface
  - The `MouseEvent` interface's `relatedTarget` attribute
  - `MouseEventInit` dictionary type
  - The `FocusEvent` interface
  - The `FocusEvent` interface's `relatedTarget` attribute
  - The `UIEvent` interface
  - The `UIEvent` interface's `view` attribute
  - The `UIEvent` interface's `detail` attribute
  - `click` event
  - `dblclick` event
  - `mousedown` event
  - `mouseenter` event
  - `mouseleave` event
  - `mousemove` event
  - `mouseout` event
  - `mouseover` event
  - `mouseup` event
  - `mousewheel` event
  - `keydown` event
  - `keyup` event
  - `keypress` event

  The following features are defined in the Touch Events specification: [TOUCH]

  - `Touch` interface
  - Touch point concept

  This specification sometimes uses the term name to refer to the event's type; as in, "an event named `click`" or "if the event name is `keypress`". The terms "name" and "type" for events are synonymous.

  The following features are defined in the DOM Parsing and Serialization specification: [DOMPARSING]

  - `DOMParser`
  - `innerHTML`
  - `outerHTML`

  The `Selection` interface is defined in the HTML Editing APIs specification. [EDITING]

  User agents are encouraged to implement the features described in the HTML Editing APIs specification. [EDITING]

  The following parts of the Fullscreen specification are referenced from this specification, in part to define the rendering of `dialog` elements, and also to define how the Fullscreen API interacts with the sandboxing features in HTML: [FULLSCREEN]

  - The top layer concept
  - `requestFullscreen()`
  - The fullscreen enabled flag
  - The fully exit fullscreen algorithm

  The High Resolution Time specification provides the `Performance` object's `now()` method. [HRT]

File API

  This specification uses the following features defined in the File API specification: [FILEAPI]

  - `Blob` interface
  - `File` interface
  - `FileList` interface
  - `Blob.close()`
  - `Blob.type`
  - The concept of read errors
Media Providers

  This specification references the following interfaces:

  - `MediaStream` [MEDIASTREAM]
  - `MediaSource` [MEDIASOURCE]
XMLHttpRequest

  This specification references the XMLHttpRequest specification to describe how the two specifications interact and to use its `ProgressEvent` features. The following features and terms are defined in the XMLHttpRequest specification: [XHR]

  - `XMLHttpRequest` interface
  - `XMLHttpRequest.responseXML` attribute
  - `ProgressEvent` interface
  - `ProgressEvent.lengthComputable` attribute
  - `ProgressEvent.loaded` attribute
  - `ProgressEvent.total` attribute
  - Fire a progress event named e
Media Queries

  Implementations must support the Media Queries language. [MQ]

CSS modules

  While support for CSS as a whole is not required of implementations of this specification (though it is encouraged, at least for Web browsers), some features are defined in terms of specific CSS requirements.

  When this specification requires that something be parsed according to a particular CSS grammar, the relevant algorithm in the CSS Syntax specification must be followed. [CSSSYNTAX]

  In particular, some features require that a string be parsed as a CSS <color> value. When parsing a CSS value, user agents are required by the CSS specifications to apply some error handling rules. These apply to this specification also. [CSSCOLOR] [CSS]

  For example, user agents are required to close all open constructs upon finding the end of a style sheet unexpectedly. Thus, when parsing the string "`rgb(0,0,0`" (with a missing close-parenthesis) for a colour value, the close parenthesis is implied by this error handling rule, and a value is obtained (the colour 'black'). However, the similar construct "`rgb(0,0,`" (with both a missing parenthesis and a missing "blue" value) cannot be parsed, as closing the open construct does not result in a viable value.

  The term named colour is defined in the CSS Color specification. [CSSCOLOR]

  The term CSS element reference identifier is used as defined in the CSS Image Values and Replaced Content specification to define the API that declares identifiers for use with the CSS 'element()' function. [CSSIMAGES]

  Similarly, the term provides a paint source is used as defined in the CSS Image Values and Replaced Content specification to define the interaction of certain HTML elements with the CSS 'element()' function. [CSSIMAGES]

  The term default object size is also defined in the CSS Image Values and Replaced Content specification. [CSSIMAGES]

  Implementations that support scripting must support the CSS Object Model. The following features and terms are defined in the CSSOM specifications: [CSSOM] [CSSOMVIEW]

  - `Screen` interface
  - `LinkStyle` interface
  - `CSSStyleDeclaration` interface
  - `cssText` attribute of `CSSStyleDeclaration`
  - `StyleSheet` interface
  - The terms create a CSS style sheet, remove a CSS style sheet, and associated CSS style sheet
  - CSS style sheets and their properties: type, location, parent CSS style sheet, owner node, owner CSS rule, media, title, alternate flag, disabled flag, CSS rules, origin-clean flag
  - Alternative style sheet sets and the preferred style sheet set
  - Serialising a CSS value
  - Scroll an element into view
  - Scroll to the beginning of the document
  - The `resize` event
  - The `scroll` event
  - The features argument of `window.open`

  The term environment encoding is defined in the CSS Syntax specifications. [CSSSYNTAX]

  The term CSS styling attribute is defined in the CSS Style Attributes specification. [CSSATTR]

  The `CanvasRenderingContext2D` object's use of fonts depends on the features described in the CSS Fonts and Font Loading specifications, including in particular `FontFace` objects and the font source concept. [CSSFONTS] [CSSFONTLOAD]

SVG

  The following interface is defined in the SVG specification: [SVG]

  - `SVGMatrix` interface
WebGL

  The following interface is defined in the WebGL specification: [WEBGL]

  - `WebGLRenderingContext` interface
WebVTT

  Implementations may support WebVTT as a text track format for subtitles, captions, chapter titles, metadata, etc, for media resources. [WEBVTT]

  The following terms, used in this specification, are defined in the WebVTT specification:

  - WebVTT file
  - WebVTT file using cue text
  - WebVTT file using chapter title text
  - WebVTT file using only nested cues
  - WebVTT parser
  - The rules for updating the display of WebVTT text tracks
  - The rules for interpreting WebVTT cue text
  - The WebVTT text track cue writing direction
  - `VTTCue` interface
The WebSocket protocol

  The following terms are defined in the WebSocket protocol specification: [WSP]

  - establish a WebSocket connection
  - the WebSocket connection is established
  - validate the server's response
  - extensions in use
  - subprotocol in use
  - headers to send appropriate cookies
  - cookies set during the server's opening handshake
  - a WebSocket message has been received
  - send a WebSocket Message
  - fail the WebSocket connection
  - close the WebSocket connection
  - start the WebSocket closing handshake
  - the WebSocket closing handshake is started
  - the WebSocket connection is closed (possibly cleanly)
  - the WebSocket connection close code
  - the WebSocket connection close reason
  - `Sec-WebSocket-Protocol` field
ARIA

  The terms strong native semantics is used as defined in the ARIA specification. The term default implicit ARIA semantics has the same meaning as the term implicit WAI-ARIA semantics as used in the ARIA specification. [ARIA]

  The `role` attribute is defined in the ARIA specification, as are the following roles: [ARIA]

  - `alert`
  - `alertdialog`
  - `application`
  - `article`
  - `banner`
  - `button`
  - `checkbox`
  - `columnheader`
  - `combobox`
  - `complementary`
  - `contentinfo`
  - `dialog`
  - `directory`
  - `document`
  - `grid`
  - `gridcell`
  - `group`
  - `heading`
  - `img`
  - `link`
  - `list`
  - `listbox`
  - `listitem`
  - `log`
  - `main`
  - `marquee`
  - `menu`
  - `menubar`
  - `menuitem`
  - `menuitemcheckbox`
  - `menuitemradio`
  - `navigation`
  - `note`
  - `option`
  - `presentation`
  - `progressbar`
  - `radio`
  - `region`
  - `row`
  - `rowgroup`
  - `rowheader`
  - `search`
  - `separator`
  - `slider`
  - `spinbutton`
  - `status`
  - `tab`
  - `tablist`
  - `textbox`
  - `toolbar`
  - `tree`
  - `treeitem`

  In addition, the following `aria-*` content attributes are defined in the ARIA specification: [ARIA]

  - `aria-checked`
  - `aria-describedby`
  - `aria-disabled`
  - `aria-expanded`
  - `aria-hidden`
  - `aria-invalid`
  - `aria-label`
  - `aria-level`
  - `aria-multiline`
  - `aria-multiselectable`
  - `aria-owns`
  - `aria-readonly`
  - `aria-required`
  - `aria-selected`
  - `aria-sort`
  - `aria-valuemax`
  - `aria-valuemin`
  - `aria-valuenow`

This specification does not require support of any particular network protocol, style sheet language, scripting language, or any of the DOM specifications beyond those required in the list above. However, the language described by this specification is biased towards CSS as the styling language, JavaScript as the scripting language, and HTTP as the network protocol, and several features assume that those languages and protocols are in use.

A user agent that implements the HTTP protocol must implement the Web Origin Concept specification and the HTTP State Management Mechanism specification (Cookies) as well. [HTTP] [ORIGIN] [COOKIES]

This specification might have certain additional requirements on character encodings, image formats, audio formats, and video formats in the respective sections.

#### 2.2.3 Extensibility

Vendor-specific proprietary user agent extensions to this specification are strongly discouraged. Documents must not use such extensions, as doing so reduces interoperability and fragments the user base, allowing only users of specific user agents to access the content in question.

If such extensions are nonetheless needed, e.g. for experimental purposes, then vendors are strongly urged to use one of the following extension mechanisms:

- For markup-level features that can be limited to the XML serialisation and need not be supported in the HTML serialisation, vendors should use the namespace mechanism to define custom namespaces in which the non-standard elements and attributes are supported.

- For markup-level features that are intended for use with the HTML syntax, extensions should be limited to new attributes of the form "`x-vendor-feature`", where vendor is a short string that identifies the vendor responsible for the extension, and feature is the name of the feature. New element names should not be created. Using attributes for such extensions exclusively allows extensions from multiple vendors to co-exist on the same element, which would not be possible with elements. Using the "`x-vendor-feature`" form allows extensions to be made without risk of conflicting with future additions to the specification.

  For instance, a browser named "FerretBrowser" could use "ferret" as a vendor prefix, while a browser named "Mellblom Browser" could use "mb". If both of these browsers invented extensions that turned elements into scratch-and-sniff areas, an author experimenting with these features could write:

      <p>This smells of lemons!
      <span x-ferret-smellovision x-ferret-smellcode="LEM01"
            x-mb-outputsmell x-mb-smell="lemon juice"></span></p>

Attribute names beginning with the two characters "`x-`" are reserved for user agent use and are guaranteed to never be formally added to the HTML language. For flexibility, attributes names containing underscores (the U+005F LOW LINE character) are also reserved for experimental purposes and are guaranteed to never be formally added to the HTML language.

Pages that use such attributes are by definition non-conforming.

For DOM extensions, e.g. new methods and IDL attributes, the new members should be prefixed by vendor-specific strings to prevent clashes with future versions of this specification.

For events, experimental event types should be prefixed with vendor-specific strings.

For example, if a user agent called "Pleasold" were to add an event to indicate when the user is going up in an elevator, it could use the prefix "`pleasold`" and thus name the event "`pleasoldgoingup`", possibly with an event handler attribute named "`onpleasoldgoingup`".

All extensions must be defined so that the use of extensions neither contradicts nor causes the non-conformance of functionality defined in the specification.

For example, while strongly discouraged from doing so, an implementation "Foo Browser" could add a new IDL attribute "`fooTypeTime`" to a control's DOM interface that returned the time it took the user to select the current value of a control (say). On the other hand, defining a new control that appears in a form's `elements` array would be in violation of the above requirement, as it would violate the definition of `elements` given in this specification.

When adding new reflecting IDL attributes corresponding to content attributes of the form "`x-vendor-feature`", the IDL attribute should be named "`vendorFeature`" (i.e. the "`x`" is dropped from the IDL attribute's name).

When vendor-neutral extensions to this specification are needed, either this specification can be updated accordingly, or an extension specification can be written that overrides the requirements in this specification. When someone applying this specification to their activities decides that they will recognise the requirements of such an extension specification, it becomes an applicable specification for the purposes of conformance requirements in this specification.

Someone could write a specification that defines any arbitrary byte stream as conforming, and then claim that their random junk is conforming. However, that does not mean that their random junk actually is conforming for everyone's purposes: if someone else decides that that specification does not apply to their work, then they can quite legitimately say that the aforementioned random junk is just that, junk, and not conforming at all. As far as conformance goes, what matters in a particular community is what that community agrees is applicable.

User agents must treat elements and attributes that they do not understand as semantically neutral; leaving them in the DOM (for DOM processors), and styling them according to CSS (for CSS processors), but not inferring any meaning from them.

When support for a feature is disabled (e.g. as an emergency measure to mitigate a security problem, or to aid in development, or for performance reasons), user agents must act as if they had no support for the feature whatsoever, and as if the feature was not mentioned in this specification. For example, if a particular feature is accessed via an attribute in a Web IDL interface, the attribute itself would be omitted from the objects that implement that interface — leaving the attribute on the object but making it return null or throw an exception is insufficient.

#### 2.2.4 Interactions with XPath and XSLT

Implementations of XPath 1.0 that operate on HTML documents parsed or created in the manners described in this specification (e.g. as part of the `document.evaluate()` API) must act as if the following edit was applied to the XPath 1.0 specification.

First, remove this paragraph:

> A QName in the node test is expanded into an expanded-name using the namespace declarations from the expression context. This is the same way expansion is done for element type names in start and end-tags except that the default namespace declared with `xmlns` is not used: if the QName does not have a prefix, then the namespace URI is null (this is the same way attribute names are expanded). It is an error if the QName has a prefix for which there is no namespace declaration in the expression context.

Then, insert in its place the following:

> A QName in the node test is expanded into an expanded-name using the namespace declarations from the expression context. If the QName has a prefix, then there must be a namespace declaration for this prefix in the expression context, and the corresponding namespace URI is the one that is associated with this prefix. It is an error if the QName has a prefix for which there is no namespace declaration in the expression context.

> If the QName has no prefix and the principal node type of the axis is element, then the default element namespace is used. Otherwise if the QName has no prefix, the namespace URI is null. The default element namespace is a member of the context for the XPath expression. The value of the default element namespace when executing an XPath expression through the DOM3 XPath API is determined in the following way:

> 1. If the context node is from an HTML DOM, the default element namespace is "http://www.w3.org/1999/xhtml".
> 2. Otherwise, the default element namespace URI is null.

> This is equivalent to adding the default element namespace feature of XPath 2.0 to XPath 1.0, and using the HTML namespace as the default element namespace for HTML documents. It is motivated by the desire to have implementations be compatible with legacy HTML content while still supporting the changes that this specification introduces to HTML regarding the namespace used for HTML elements, and by the desire to use XPath 1.0 rather than XPath 2.0.

This change is a willful violation of the XPath 1.0 specification, motivated by desire to have implementations be compatible with legacy content while still supporting the changes that this specification introduces to HTML regarding which namespace is used for HTML elements. [XPATH10]

XSLT 1.0 processors outputting to a DOM when the output method is "html" (either explicitly or via the defaulting rule in XSLT 1.0) are affected as follows:

If the transformation program outputs an element in no namespace, the processor must, prior to constructing the corresponding DOM element node, change the namespace of the element to the HTML namespace, ASCII-lowercase the element's local name, and ASCII-lowercase the names of any non-namespaced attributes on the element.

This requirement is a willful violation of the XSLT 1.0 specification, required because this specification changes the namespaces and case-sensitivity rules of HTML in a manner that would otherwise be incompatible with DOM-based XSLT transformations. (Processors that serialise the output are unaffected.) [XSLT10]

This specification does not specify precisely how XSLT processing interacts with the HTML parser infrastructure (for example, whether an XSLT processor acts as if it puts any elements into a stack of open elements). However, XSLT processors must stop parsing if they successfully complete, and must set the current document readiness first to "`interactive`" and then to "`complete`" if they are aborted.

This specification does not specify how XSLT interacts with the navigation algorithm, how it fits in with the event loop, nor how error pages are to be handled (e.g. whether XSLT errors are to replace an incremental XSLT output, or are rendered inline, etc).

There are also additional non-normative comments regarding the interaction of XSLT and HTML in the `script` element section, and of XSLT, XPath, and HTML in the `template` element section.

### 2.3 Case-sensitivity and string comparison

Comparing two strings in a case-sensitive manner means comparing them exactly, code point for code point.

Comparing two strings in an ASCII case-insensitive manner means comparing them exactly, code point for code point, except that the characters in the range U+0041 to U+005A (i.e. LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z) and the corresponding characters in the range U+0061 to U+007A (i.e. LATIN SMALL LETTER A to LATIN SMALL LETTER Z) are considered to also match.

Comparing two strings in a compatibility caseless manner means using the Unicode compatibility caseless match operation to compare the two strings, with no language-specific tailoirings. [UNICODE]

Except where otherwise stated, string comparisons must be performed in a case-sensitive manner.

Converting a string to ASCII uppercase means replacing all characters in the range U+0061 to U+007A (i.e. LATIN SMALL LETTER A to LATIN SMALL LETTER Z) with the corresponding characters in the range U+0041 to U+005A (i.e. LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z).

Converting a string to ASCII lowercase means replacing all characters in the range U+0041 to U+005A (i.e. LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z) with the corresponding characters in the range U+0061 to U+007A (i.e. LATIN SMALL LETTER A to LATIN SMALL LETTER Z).

A string pattern is a prefix match for a string s when pattern is not longer than s and truncating s to pattern's length leaves the two strings as matches of each other.

### 2.4 Common microsyntaxes

There are various places in HTML that accept particular data types, such as dates or numbers. This section describes what the conformance criteria for content in those formats is, and how to parse them.

Implementors are strongly urged to carefully examine any third-party libraries they might consider using to implement the parsing of syntaxes described below. For example, date libraries are likely to implement error handling behaviour that differs from what is required in this specification, since error-handling behaviour is often not defined in specifications that describe date syntaxes similar to those used in this specification, and thus implementations tend to vary greatly in how they handle errors.

#### 2.4.1 Common parser idioms

The space characters, for the purposes of this specification, are U+0020 SPACE, U+0009 CHARACTER TABULATION (tab), U+000A LINE FEED (LF), U+000C FORM FEED (FF), and U+000D CARRIAGE RETURN (CR).

The White_Space characters are those that have the Unicode property "White_Space" in the Unicode `PropList.txt` data file. [UNICODE]

This should not be confused with the "White_Space" value (abbreviated "WS") of the "Bidi_Class" property in the `Unicode.txt` data file.

The control characters are those whose Unicode "General_Category" property has the value "Cc" in the Unicode `UnicodeData.txt` data file. [UNICODE]

The uppercase ASCII letters are the characters in the range U+0041 LATIN CAPITAL LETTER A to U+005A LATIN CAPITAL LETTER Z.

The lowercase ASCII letters are the characters in the range U+0061 LATIN SMALL LETTER A to U+007A LATIN SMALL LETTER Z.

The ASCII digits are the characters in the range U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9).

The alphanumeric ASCII characters are those that are either uppercase ASCII letters, lowercase ASCII letters, or ASCII digits.

The ASCII hex digits are the characters in the ranges U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), U+0041 LATIN CAPITAL LETTER A to U+0046 LATIN CAPITAL LETTER F, and U+0061 LATIN SMALL LETTER A to U+0066 LATIN SMALL LETTER F.

The uppercase ASCII hex digits are the characters in the ranges U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9) and U+0041 LATIN CAPITAL LETTER A to U+0046 LATIN CAPITAL LETTER F only.

The lowercase ASCII hex digits are the characters in the ranges U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9) and U+0061 LATIN SMALL LETTER A to U+0066 LATIN SMALL LETTER F only.

Some of the micro-parsers described below follow the pattern of having an input variable that holds the string being parsed, and having a position variable pointing at the next character to parse in input.

For parsers based on this pattern, a step that requires the user agent to collect a sequence of characters means that the following algorithm must be run, with characters being the set of characters that can be collected:

1. Let input and position be the same variables as those of the same name in the algorithm that invoked these steps.

2. Let result be the empty string.

3. While position doesn't point past the end of input and the character at position is one of the characters, append that character to the end of result and advance position to the next character in input.

4. Return result.

The step skip whitespace means that the user agent must collect a sequence of characters that are space characters. The collected characters are not used.

When a user agent is to strip line breaks from a string, the user agent must remove any U+000A LINE FEED (LF) and U+000D CARRIAGE RETURN (CR) characters from that string.

When a user agent is to strip leading and trailing whitespace from a string, the user agent must remove all space characters that are at the start or end of the string.

When a user agent is to strip and collapse whitespace in a string, it must replace any sequence of one or more consecutive space characters in that string with a single U+0020 SPACE character, and then strip leading and trailing whitespace from that string.

When a user agent has to strictly split a string on a particular delimiter character delimiter, it must use the following algorithm:

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Let tokens be an ordered list of tokens, initially empty.

4. While position is not past the end of input:

   1. Collect a sequence of characters that are not the delimiter character.

   2. Append the string collected in the previous step to tokens.

   3. Advance position to the next character in input.

5. Return tokens.

For the special cases of splitting a string on spaces and on commas, this algorithm does not apply (those algorithms also perform whitespace trimming).

#### 2.4.2 Boolean attributes

A number of attributes are boolean attributes. The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.

If the attribute is present, its value must either be the empty string or a value that is an ASCII case-insensitive match for the attribute's canonical name, with no leading or trailing whitespace.

The values "true" and "false" are not allowed on boolean attributes. To represent a false value, the attribute has to be omitted altogether.

Here is an example of a checkbox that is checked and disabled. The `checked` and `disabled` attributes are the boolean attributes.

    <label><input type=checkbox checked name=cheese disabled> Cheese</label>

This could be equivalently written as this:

    <label><input type=checkbox checked=checked name=cheese disabled=disabled> Cheese</label>

You can also mix styles; the following is still equivalent:

    <label><input type='checkbox' checked name=cheese disabled=""> Cheese</label>

#### 2.4.3 Keywords and enumerated attributes

Some attributes are defined as taking one of a finite set of keywords. Such attributes are called enumerated attributes. The keywords are each defined to map to a particular state (several keywords might map to the same state, in which case some of the keywords are synonyms of each other; additionally, some of the keywords can be said to be non-conforming, and are only in the specification for historical reasons). In addition, two default states can be given. The first is the invalid value default, the second is the missing value default.

If an enumerated attribute is specified, the attribute's value must be an ASCII case-insensitive match for one of the given keywords that are not said to be non-conforming, with no leading or trailing whitespace.

When the attribute is specified, if its value is an ASCII case-insensitive match for one of the given keywords then that keyword's state is the state that the attribute represents. If the attribute value matches none of the given keywords, but the attribute has an invalid value default, then the attribute represents that state. Otherwise, if the attribute value matches none of the keywords but there is a missing value default state defined, then that is the state represented by the attribute. Otherwise, there is no default, and invalid values mean that there is no state represented.

When the attribute is not specified, if there is a missing value default state defined, then that is the state represented by the (missing) attribute. Otherwise, the absence of the attribute means that there is no state represented.

The empty string can be a valid keyword.

#### 2.4.4 Numbers

##### 2.4.4.1 Signed integers

A string is a valid integer if it consists of one or more ASCII digits, optionally prefixed with a U+002D HYPHEN-MINUS character (-).

A valid integer without a U+002D HYPHEN-MINUS (-) prefix represents the number that is represented in base ten by that string of digits. A valid integer with a U+002D HYPHEN-MINUS (-) prefix represents the number represented in base ten by the string of digits that follows the U+002D HYPHEN-MINUS, subtracted from zero.

The rules for parsing integers are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either an integer or an error.

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Let sign have the value "positive".

4. Skip whitespace.

5. If position is past the end of input, return an error.

6. If the character indicated by position (the first character) is a U+002D HYPHEN-MINUS character (-):

   1. Let sign be "negative".
   2. Advance position to the next character.
   3. If position is past the end of input, return an error.

   Otherwise, if the character indicated by position (the first character) is a U+002B PLUS SIGN character (+):

   1. Advance position to the next character. (The "`+`" is ignored, but it is not conforming.)
   2. If position is past the end of input, return an error.

7. If the character indicated by position is not an ASCII digit, then return an error.

8. Collect a sequence of characters that are ASCII digits, and interpret the resulting sequence as a base-ten integer. Let value be that integer.

9. If sign is "positive", return value, otherwise return the result of subtracting value from zero.

##### 2.4.4.2 Non-negative integers

A string is a valid non-negative integer if it consists of one or more ASCII digits.

A valid non-negative integer represents the number that is represented in base ten by that string of digits.

The rules for parsing non-negative integers are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either zero, a positive integer, or an error.

1. Let input be the string being parsed.

2. Let value be the result of parsing input using the rules for parsing integers.

3. If value is an error, return an error.

4. If value is less than zero, return an error.

5. Return value.

##### 2.4.4.3 Floating-point numbers

A string is a valid floating-point number if it consists of:

1. Optionally, a U+002D HYPHEN-MINUS character (-).
2. One or both of the following, in the given order:
   1. A series of one or more ASCII digits.
   2. 1. A single U+002E FULL STOP character (.).
      2. A series of one or more ASCII digits.
3. Optionally:
   1. Either a U+0065 LATIN SMALL LETTER E character (e) or a U+0045 LATIN CAPITAL LETTER E character (E).
   2. Optionally, a U+002D HYPHEN-MINUS character (-) or U+002B PLUS SIGN character (+).
   3. A series of one or more ASCII digits.

A valid floating-point number represents the number obtained by multiplying the significand by ten raised to the power of the exponent, where the significand is the first number, interpreted as base ten (including the decimal point and the number after the decimal point, if any, and interpreting the significand as a negative number if the whole string starts with a U+002D HYPHEN-MINUS character (-) and the number is not zero), and where the exponent is the number after the E, if any (interpreted as a negative number if there is a U+002D HYPHEN-MINUS character (-) between the E and the number and the number is not zero, or else ignoring a U+002B PLUS SIGN character (+) between the E and the number if there is one). If there is no E, then the exponent is treated as zero.

The Infinity and Not-a-Number (NaN) values are not valid floating-point numbers.

The best representation of the number n as a floating-point number is the string obtained from applying the JavaScript operator ToString to n. The JavaScript operator ToString is not uniquely determined. When there are multiple possible strings that could be obtained from the JavaScript operator ToString for a particular value, the user agent must always return the same string for that value (though it may differ from the value used by other user agents).

The rules for parsing floating-point number values are as given in the following algorithm. This algorithm must be aborted at the first step that returns something. This algorithm will return either a number or an error.

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Let value have the value 1.

4. Let divisor have the value 1.

5. Let exponent have the value 1.

6. Skip whitespace.

7. If position is past the end of input, return an error.

8. If the character indicated by position is a U+002D HYPHEN-MINUS character (-):

   1. Change value and divisor to −1.
   2. Advance position to the next character.
   3. If position is past the end of input, return an error.

   Otherwise, if the character indicated by position (the first character) is a U+002B PLUS SIGN character (+):

   1. Advance position to the next character. (The "`+`" is ignored, but it is not conforming.)
   2. If position is past the end of input, return an error.

9. If the character indicated by position is a U+002E FULL STOP (.), and that is not the last character in input, and the character after the character indicated by position is an ASCII digit, then set value to zero and jump to the step labeled fraction.

10. If the character indicated by position is not an ASCII digit, then return an error.

11. Collect a sequence of characters that are ASCII digits, and interpret the resulting sequence as a base-ten integer. Multiply value by that integer.

12. If position is past the end of input, jump to the step labeled conversion.

13. Fraction: If the character indicated by position is a U+002E FULL STOP (.), run these substeps:

    1. Advance position to the next character.

    2. If position is past the end of input, or if the character indicated by position is not an ASCII digit, U+0065 LATIN SMALL LETTER E (e), or U+0045 LATIN CAPITAL LETTER E (E), then jump to the step labeled conversion.

    3. If the character indicated by position is a U+0065 LATIN SMALL LETTER E character (e) or a U+0045 LATIN CAPITAL LETTER E character (E), skip the remainder of these substeps.

    4. Fraction loop: Multiply divisor by ten.

    5. Add the value of the character indicated by position, interpreted as a base-ten digit (0..9) and divided by divisor, to value.

    6. Advance position to the next character.

    7. If position is past the end of input, then jump to the step labeled conversion.

    8. If the character indicated by position is an ASCII digit, jump back to the step labeled fraction loop in these substeps.

14. If the character indicated by position is a U+0065 LATIN SMALL LETTER E character (e) or a U+0045 LATIN CAPITAL LETTER E character (E), run these substeps:

    1. Advance position to the next character.

    2. If position is past the end of input, then jump to the step labeled conversion.

    3. If the character indicated by position is a U+002D HYPHEN-MINUS character (-):

       1. Change exponent to −1.
       2. Advance position to the next character.

       3. If position is past the end of input, then jump to the step labeled conversion.

       Otherwise, if the character indicated by position is a U+002B PLUS SIGN character (+):

       1. Advance position to the next character.

       2. If position is past the end of input, then jump to the step labeled conversion.

    4. If the character indicated by position is not an ASCII digit, then jump to the step labeled conversion.

    5. Collect a sequence of characters that are ASCII digits, and interpret the resulting sequence as a base-ten integer. Multiply exponent by that integer.

    6. Multiply value by ten raised to the exponentth power.

15. Conversion: Let S be the set of finite IEEE 754 double-precision floating-point values except −0, but with two special values added: 21024 and −21024.

16. Let rounded-value be the number in S that is closest to value, selecting the number with an even significand if there are two equally close values. (The two special values 21024 and −21024 are considered to have even significands for this purpose.)

17. If rounded-value is 21024 or −21024, return an error.

18. Return rounded-value.

##### 2.4.4.4 Percentages and lengths

The rules for parsing dimension values are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either a number greater than or equal to 1.0, or an error; if a number is returned, then it is further categorised as either a percentage or a length.

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Skip whitespace.

4. If position is past the end of input, return an error.

5. If the character indicated by position is a U+002B PLUS SIGN character (+), advance position to the next character.

6. Collect a sequence of characters that are U+0030 DIGIT ZERO (0) characters, and discard them.

7. If position is past the end of input, return an error.

8. If the character indicated by position is not one of U+0031 DIGIT ONE (1) to U+0039 DIGIT NINE (9), then return an error.

9. Collect a sequence of characters that are ASCII digits, and interpret the resulting sequence as a base-ten integer. Let value be that number.

10. If position is past the end of input, return value as a length.

11. If the character indicated by position is a U+002E FULL STOP character (.):

    1. Advance position to the next character.

    2. If position is past the end of input, or if the character indicated by position is not an ASCII digit, then return value as a length.

    3. Let divisor have the value 1.

    4. Fraction loop: Multiply divisor by ten.

    5. Add the value of the character indicated by position, interpreted as a base-ten digit (0..9) and divided by divisor, to value.

    6. Advance position to the next character.

    7. If position is past the end of input, then return value as a length.

    8. If the character indicated by position is an ASCII digit, return to the step labeled fraction loop in these substeps.

12. If position is past the end of input, return value as a length.

13. If the character indicated by position is a U+0025 PERCENT SIGN character (%), return value as a percentage.

14. Return value as a length.

##### 2.4.4.5 Lists of integers

A valid list of integers is a number of valid integers separated by U+002C COMMA characters, with no other characters (e.g. no space characters). In addition, there might be restrictions on the number of integers that can be given, or on the range of values allowed.

The rules for parsing a list of integers are as follows:

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Let numbers be an initially empty list of integers. This list will be the result of this algorithm.

4. If there is a character in the string input at position position, and it is either a U+0020 SPACE, U+002C COMMA, or U+003B SEMICOLON character, then advance position to the next character in input, or to beyond the end of the string if there are no more characters.

5. If position points to beyond the end of input, return numbers and abort.

6. If the character in the string input at position position is a U+0020 SPACE, U+002C COMMA, or U+003B SEMICOLON character, then return to step 4.

7. Let negated be false.

8. Let value be 0.

9. Let started be false. This variable is set to true when the parser sees a number or a U+002D HYPHEN-MINUS character (-).

10. Let got number be false. This variable is set to true when the parser sees a number.

11. Let finished be false. This variable is set to true to switch parser into a mode where it ignores characters until the next separator.

12. Let bogus be false.

13. Parser: If the character in the string input at position position is:

    A U+002D HYPHEN-MINUS character

      Follow these substeps:

      1. If got number is true, let finished be true.
      2. If finished is true, skip to the next step in the overall set of steps.
      3. If started is true, let negated be false.
      4. Otherwise, if started is false and if bogus is false, let negated be true.
      5. Let started be true.
    An ASCII digit

      Follow these substeps:

      1. If finished is true, skip to the next step in the overall set of steps.
      2. Multiply value by ten.
      3. Add the value of the digit, interpreted in base ten, to value.
      4. Let started be true.
      5. Let got number be true.
    A U+0020 SPACE character
    A U+002C COMMA character
    A U+003B SEMICOLON character

      Follow these substeps:

      1. If got number is false, return the numbers list and abort. This happens if an entry in the list has no digits, as in "`1,2,x,4`".
      2. If negated is true, then negate value.
      3. Append value to the numbers list.
      4. Jump to step 4 in the overall set of steps.
    A character in the range U+0001 to U+001F, U+0021 to U+002B, U+002D to U+002F, U+003A, U+003C to U+0040, U+005B to U+0060, U+007b to U+007F (i.e. any other non-alphabetic ASCII character)

      Follow these substeps:

      1. If got number is true, let finished be true.
      2. If finished is true, skip to the next step in the overall set of steps.
      3. Let negated be false.
    Any other character

      Follow these substeps:

      1. If finished is true, skip to the next step in the overall set of steps.
      2. Let negated be false.
      3. Let bogus be true.
      4. If started is true, then return the numbers list, and abort. (The value in value is not appended to the list first; it is dropped.)

14. Advance position to the next character in input, or to beyond the end of the string if there are no more characters.

15. If position points to a character (and not to beyond the end of input), jump to the big Parser step above.

16. If negated is true, then negate value.

17. If got number is true, then append value to the numbers list.

18. Return the numbers list and abort.

##### 2.4.4.6 Lists of dimensions

The rules for parsing a list of dimensions are as follows. These rules return a list of zero or more pairs consisting of a number and a unit, the unit being one of percentage, relative, and absolute.

1. Let raw input be the string being parsed.

2. If the last character in raw input is a U+002C COMMA character (,), then remove that character from raw input.

3. Split the string raw input on commas. Let raw tokens be the resulting list of tokens.

4. Let result be an empty list of number/unit pairs.

5. For each token in raw tokens, run the following substeps:

   1. Let input be the token.

   2. Let position be a pointer into input, initially pointing at the start of the string.

   3. Let value be the number 0.

   4. Let unit be absolute.

   5. If position is past the end of input, set unit to relative and jump to the last substep.

   6. If the character at position is an ASCII digit, collect a sequence of characters that are ASCII digits, interpret the resulting sequence as an integer in base ten, and increment value by that integer.

   7. If the character at position is a U+002E FULL STOP character (.), run these substeps:

      1. Collect a sequence of characters consisting of space characters and ASCII digits. Let s be the resulting sequence.

      2. Remove all space characters in s.

      3. If s is not the empty string, run these subsubsteps:

         1. Let length be the number of characters in s (after the spaces were removed).

         2. Let fraction be the result of interpreting s as a base-ten integer, and then dividing that number by 10length.

         3. Increment value by fraction.

   8. Skip whitespace.

   9. If the character at position is a U+0025 PERCENT SIGN character (%), then set unit to percentage.

      Otherwise, if the character at position is a U+002A ASTERISK character (*), then set unit to relative.

   10. Add an entry to result consisting of the number given by value and the unit given by unit.

6. Return the list result.

#### 2.4.5 Dates and times

In the algorithms below, the number of days in month month of year year is: 31 if month is 1, 3, 5, 7, 8, 10, or 12; 30 if month is 4, 6, 9, or 11; 29 if month is 2 and year is a number divisible by 400, or if year is a number divisible by 4 but not by 100; and 28 otherwise. This takes into account leap years in the Gregorian calendar. [GREGORIAN]

When ASCII digits are used in the date and time syntaxes defined in this section, they express numbers in base ten.

While the formats described here are intended to be subsets of the corresponding ISO8601 formats, this specification defines parsing rules in much more detail than ISO8601. Implementors are therefore encouraged to carefully examine any date parsing libraries before using them to implement the parsing rules described below; ISO8601 libraries might not parse dates and times in exactly the same manner. [ISO8601]

Where this specification refers to the proleptic Gregorian calendar, it means the modern Gregorian calendar, extrapolated backwards to year 1. A date in the proleptic Gregorian calendar, sometimes explicitly referred to as a proleptic-Gregorian date, is one that is described using that calendar even if that calendar was not in use at the time (or place) in question. [GREGORIAN]

The use of the Gregorian calendar as the wire format in this specification is an arbitrary choice resulting from the cultural biases of those involved in the decision. See also the section discussing date, time, and number formats in forms (for authors), implemention notes regarding localization of form controls, and the `time` element.

##### 2.4.5.1 Months

A month consists of a specific proleptic-Gregorian date with no time-zone information and no date information beyond a year and a month. [GREGORIAN]

A string is a valid month string representing a year year and month month if it consists of the following components in the given order:

1. Four or more ASCII digits, representing year, where year > 0
2. A U+002D HYPHEN-MINUS character (-)
3. Two ASCII digits, representing the month month, in the range 1 ≤ month ≤ 12

The rules to parse a month string are as follows. This will return either a year and month, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Parse a month component to obtain year and month. If this returns nothing, then fail.

4. If position is not beyond the end of input, then fail.

5. Return year and month.

The rules to parse a month component, given an input string and a position, are as follows. This will return either a year and a month, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

1. Collect a sequence of characters that are ASCII digits. If the collected sequence is not at least four characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the year.

2. If year is not a number greater than zero, then fail.

3. If position is beyond the end of input or if the character at position is not a U+002D HYPHEN-MINUS character, then fail. Otherwise, move position forwards one character.

4. Collect a sequence of characters that are ASCII digits. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the month.

5. If month is not a number in the range 1 ≤ month ≤ 12, then fail.

6. Return year and month.

##### 2.4.5.2 Dates

A date consists of a specific proleptic-Gregorian date with no time-zone information, consisting of a year, a month, and a day. [GREGORIAN]

A string is a valid date string representing a year year, month month, and day day if it consists of the following components in the given order:

1. A valid month string, representing year and month
2. A U+002D HYPHEN-MINUS character (-)
3. Two ASCII digits, representing day, in the range 1 ≤ day ≤ maxday where maxday is the number of days in the month month and year year

The rules to parse a date string are as follows. This will return either a date, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Parse a date component to obtain year, month, and day. If this returns nothing, then fail.

4. If position is not beyond the end of input, then fail.

5. Let date be the date with year year, month month, and day day.

6. Return date.

The rules to parse a date component, given an input string and a position, are as follows. This will return either a year, a month, and a day, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

1. Parse a month component to obtain year and month. If this returns nothing, then fail.

2. Let maxday be the number of days in month month of year year.

3. If position is beyond the end of input or if the character at position is not a U+002D HYPHEN-MINUS character, then fail. Otherwise, move position forwards one character.

4. Collect a sequence of characters that are ASCII digits. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the day.

5. If day is not a number in the range 1 ≤ day ≤ maxday, then fail.

6. Return year, month, and day.

##### 2.4.5.3 Yearless dates

A yearless date consists of a Gregorian month and a day within that month, but with no associated year. [GREGORIAN]

A string is a valid yearless date string representing a month month and a day day if it consists of the following components in the given order:

1. Optionally, two U+002D HYPHEN-MINUS characters (-)
2. Two ASCII digits, representing the month month, in the range 1 ≤ month ≤ 12
3. A U+002D HYPHEN-MINUS character (-)
4. Two ASCII digits, representing day, in the range 1 ≤ day ≤ maxday where maxday is the number of days in the month month and any arbitrary leap year (e.g. 4 or 2000)

In other words, if the month is "`02`", meaning February, then the day can be 29, as if the year was a leap year.

The rules to parse a yearless date string are as follows. This will return either a month and a day, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Parse a yearless date component to obtain month and day. If this returns nothing, then fail.

4. If position is not beyond the end of input, then fail.

5. Return month and day.

The rules to parse a yearless date component, given an input string and a position, are as follows. This will return either a month and a day, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

1. Collect a sequence of characters that are U+002D HYPHEN-MINUS characters (-). If the collected sequence is not exactly zero or two characters long, then fail.

2. Collect a sequence of characters that are ASCII digits. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the month.

3. If month is not a number in the range 1 ≤ month ≤ 12, then fail.

4. Let maxday be the number of days in month month of any arbitrary leap year (e.g. 4 or 2000).

5. If position is beyond the end of input or if the character at position is not a U+002D HYPHEN-MINUS character, then fail. Otherwise, move position forwards one character.

6. Collect a sequence of characters that are ASCII digits. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the day.

7. If day is not a number in the range 1 ≤ day ≤ maxday, then fail.

8. Return month and day.

##### 2.4.5.4 Times

A time consists of a specific time with no time-zone information, consisting of an hour, a minute, a second, and a fraction of a second.

A string is a valid time string representing an hour hour, a minute minute, and a second second if it consists of the following components in the given order:

1. Two ASCII digits, representing hour, in the range 0 ≤ hour ≤ 23
2. A U+003A COLON character (:)
3. Two ASCII digits, representing minute, in the range 0 ≤ minute ≤ 59
4. If second is non-zero, or optionally if second is zero:
   1. A U+003A COLON character (:)
   2. Two ASCII digits, representing the integer part of second, in the range 0 ≤ s ≤ 59
   3. If second is not an integer, or optionally if second is an integer:
      1. A 002E FULL STOP character (.)
      2. One, two, or three ASCII digits, representing the fractional part of second

The second component cannot be 60 or 61; leap seconds cannot be represented.

The rules to parse a time string are as follows. This will return either a time, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Parse a time component to obtain hour, minute, and second. If this returns nothing, then fail.

4. If position is not beyond the end of input, then fail.

5. Let time be the time with hour hour, minute minute, and second second.

6. Return time.

The rules to parse a time component, given an input string and a position, are as follows. This will return either an hour, a minute, and a second, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

1. Collect a sequence of characters that are ASCII digits. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the hour.

2. If hour is not a number in the range 0 ≤ hour ≤ 23, then fail.

3. If position is beyond the end of input or if the character at position is not a U+003A COLON character, then fail. Otherwise, move position forwards one character.

4. Collect a sequence of characters that are ASCII digits. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the minute.

5. If minute is not a number in the range 0 ≤ minute ≤ 59, then fail.

6. Let second be a string with the value "0".

7. If position is not beyond the end of input and the character at position is a U+003A COLON, then run these substeps:

   1. Advance position to the next character in input.

   2. If position is beyond the end of input, or at the last character in input, or if the next two characters in input starting at position are not both ASCII digits, then fail.

   3. Collect a sequence of characters that are either ASCII digits or U+002E FULL STOP characters. If the collected sequence is three characters long, or if it is longer than three characters long and the third character is not a U+002E FULL STOP character, or if it has more than one U+002E FULL STOP character, then fail. Otherwise, let the collected string be second instead of its previous value.

8. Interpret second as a base-ten number (possibly with a fractional part). Let second be that number instead of the string version.

9. If second is not a number in the range 0 ≤ second < 60, then fail.

10. Return hour, minute, and second.

##### 2.4.5.5 Local dates and times

A local date and time consists of a specific proleptic-Gregorian date, consisting of a year, a month, and a day, and a time, consisting of an hour, a minute, a second, and a fraction of a second, but expressed without a time zone. [GREGORIAN]

A string is a valid local date and time string representing a date and time if it consists of the following components in the given order:

1. A valid date string representing the date
2. A U+0054 LATIN CAPITAL LETTER T character (T) or a U+0020 SPACE character
3. A valid time string representing the time

A string is a valid normalised local date and time string representing a date and time if it consists of the following components in the given order:

1. A valid date string representing the date
2. A U+0054 LATIN CAPITAL LETTER T character (T)
3. A valid time string representing the time, expressed as the shortest possible string for the given time (e.g. omitting the seconds component entirely if the given time is zero seconds past the minute)

The rules to parse a local date and time string are as follows. This will return either a date and time, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Parse a date component to obtain year, month, and day. If this returns nothing, then fail.

4. If position is beyond the end of input or if the character at position is neither a U+0054 LATIN CAPITAL LETTER T character (T) nor a U+0020 SPACE character, then fail. Otherwise, move position forwards one character.

5. Parse a time component to obtain hour, minute, and second. If this returns nothing, then fail.

6. If position is not beyond the end of input, then fail.

7. Let date be the date with year year, month month, and day day.

8. Let time be the time with hour hour, minute minute, and second second.

9. Return date and time.

##### 2.4.5.6 Time zones

A time-zone offset consists of a signed number of hours and minutes.

A string is a valid time-zone offset string representing a time-zone offset if it consists of either:

- A U+005A LATIN CAPITAL LETTER Z character (Z), allowed only if the time zone is UTC

- Or, the following components, in the given order:

  1. Either a U+002B PLUS SIGN character (+) or, if the time-zone offset is not zero, a U+002D HYPHEN-MINUS character (-), representing the sign of the time-zone offset
  2. Two ASCII digits, representing the hours component hour of the time-zone offset, in the range 0 ≤ hour ≤ 23
  3. Optionally, a U+003A COLON character (:)
  4. Two ASCII digits, representing the minutes component minute of the time-zone offset, in the range 0 ≤ minute ≤ 59

This format allows for time-zone offsets from -23:59 to +23:59. Right now, in practice, the range of offsets of actual time zones is -12:00 to +14:00, and the minutes component of offsets of actual time zones is always either 00, 30, or 45. There is no guarantee that this will remain so forever, however, since time zones are used as political footballs and are thus subject to very whimsical policy decisions.

See also the usage notes and examples in the global date and time section below for details on using time-zone offsets with historical times that predate the formation of formal time zones.

The rules to parse a time-zone offset string are as follows. This will return either a time-zone offset, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Parse a time-zone offset component to obtain timezonehours and timezoneminutes. If this returns nothing, then fail.

4. If position is not beyond the end of input, then fail.

5. Return the time-zone offset that is timezonehours hours and timezoneminutes minutes from UTC.

The rules to parse a time-zone offset component, given an input string and a position, are as follows. This will return either time-zone hours and time-zone minutes, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

1. If the character at position is a U+005A LATIN CAPITAL LETTER Z character (Z), then:

   1. Let timezonehours be 0.

   2. Let timezoneminutes be 0.

   3. Advance position to the next character in input.

   Otherwise, if the character at position is either a U+002B PLUS SIGN (+) or a U+002D HYPHEN-MINUS (-), then:

   1. If the character at position is a U+002B PLUS SIGN (+), let sign be "positive". Otherwise, it's a U+002D HYPHEN-MINUS (-); let sign be "negative".

   2. Advance position to the next character in input.

   3. Collect a sequence of characters that are ASCII digits. Let s be the collected sequence.

   4. If s is exactly two characters long, then run these substeps:

      1. Interpret s as a base-ten integer. Let that number be the timezonehours.

      2. If position is beyond the end of input or if the character at position is not a U+003A COLON character, then fail. Otherwise, move position forwards one character.

      3. Collect a sequence of characters that are ASCII digits. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the timezoneminutes.

      If s is exactly four characters long, then run these substeps:

      1. Interpret the first two characters of s as a base-ten integer. Let that number be the timezonehours.

      2. Interpret the last two characters of s as a base-ten integer. Let that number be the timezoneminutes.

      Otherwise, fail.

   5. If timezonehours is not a number in the range 0 ≤ timezonehours ≤ 23, then fail.
   6. If sign is "negative", then negate timezonehours.
   7. If timezoneminutes is not a number in the range 0 ≤ timezoneminutes ≤ 59, then fail.
   8. If sign is "negative", then negate timezoneminutes.

   Otherwise, fail.

2. Return timezonehours and timezoneminutes.

##### 2.4.5.7 Global dates and times

A global date and time consists of a specific proleptic-Gregorian date, consisting of a year, a month, and a day, and a time, consisting of an hour, a minute, a second, and a fraction of a second, expressed with a time-zone offset, consisting of a signed number of hours and minutes. [GREGORIAN]

A string is a valid global date and time string representing a date, time, and a time-zone offset if it consists of the following components in the given order:

1. A valid date string representing the date
2. A U+0054 LATIN CAPITAL LETTER T character (T) or a U+0020 SPACE character
3. A valid time string representing the time
4. A valid time-zone offset string representing the time-zone offset

Times in dates before the formation of UTC in the mid twentieth century must be expressed and interpreted in terms of UT1 (contemporary Earth solar time at the 0° longitude), not UTC (the approximation of UT1 that ticks in SI seconds). Time before the formation of time zones must be expressed and interpeted as UT1 times with explicit time zones that approximate the contemporary difference between the appropriate local time and the time observed at the location of Greenwich, London.

The following are some examples of dates written as valid global date and time strings.

"`0037-12-13 00:00Z`"
  Midnight in areas using London time on the birthday of Nero (the Roman Emperor). See below for further discussion on which date this actually corresponds to.
"`1979-10-14T12:00:00.001-04:00`"
  One millisecond after noon on October 14th 1979, in the time zone in use on the east coast of the USA during daylight saving time.
"`8592-01-01T02:09+02:09`"
  Midnight UTC on the 1st of January, 8592. The time zone associated with that time is two hours and nine minutes ahead of UTC, which is not currently a real time zone, but is nonetheless allowed.

Several things are notable about these dates:

- Years with fewer than four digits have to be zero-padded. The date "37-12-13" would not be a valid date.
- If the "`T`" is replaced by a space, it must be a single space character. The string "`2001-12-21 12:00Z`" (with two spaces between the components) would not be parsed successfully.
- To unambiguously identify a moment in time prior to the introduction of the Gregorian calendar (insofar as moments in time before the formation of UTC can be unambiguously identified), the date has to be first converted to the Gregorian calendar from the calendar in use at the time (e.g. from the Julian calendar). The date of Nero's birth is the 15th of December 37, in the Julian Calendar, which is the 13th of December 37 in the proleptic Gregorian calendar.
- The time and time-zone offset components are not optional.
- Dates before the year one can't be represented as a datetime in this version of HTML.
- Times of specific events in ancient times are, at best, approximations, since time was not well coordinated or measured until relatively recent decades.
- Time-zone offsets differ based on daylight savings time.

A string is a valid normalised forced-UTC global date and time string representing a date, time, and a time-zone offset if it consists of the following components in the given order:

1. A valid date string representing the date converted to the UTC time zone
2. A U+0054 LATIN CAPITAL LETTER T character (T)
3. A valid time string representing the time converted to the UTC time zone and expressed as the shortest possible string for the given time (e.g. omitting the seconds component entirely if the given time is zero seconds past the minute)
4. A U+005A LATIN CAPITAL LETTER Z character (Z)

The rules to parse a global date and time string are as follows. This will return either a time in UTC, with associated time-zone offset information for round-tripping or display purposes, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Parse a date component to obtain year, month, and day. If this returns nothing, then fail.

4. If position is beyond the end of input or if the character at position is neither a U+0054 LATIN CAPITAL LETTER T character (T) nor a U+0020 SPACE character, then fail. Otherwise, move position forwards one character.

5. Parse a time component to obtain hour, minute, and second. If this returns nothing, then fail.

6. If position is beyond the end of input, then fail.

7. Parse a time-zone offset component to obtain timezonehours and timezoneminutes. If this returns nothing, then fail.

8. If position is not beyond the end of input, then fail.

9. Let time be the moment in time at year year, month month, day day, hours hour, minute minute, second second, subtracting timezonehours hours and timezoneminutes minutes. That moment in time is a moment in the UTC time zone.

10. Let timezone be timezonehours hours and timezoneminutes minutes from UTC.

11. Return time and timezone.

##### 2.4.5.8 Weeks

A week consists of a week-year number and a week number representing a seven-day period starting on a Monday. Each week-year in this calendaring system has either 52 or 53 such seven-day periods, as defined below. The seven-day period starting on the Gregorian date Monday December 29th 1969 (1969-12-29) is defined as week number 1 in week-year 1970. Consecutive weeks are numbered sequentially. The week before the number 1 week in a week-year is the last week in the previous week-year, and vice versa. [GREGORIAN]

A week-year with a number year has 53 weeks if it corresponds to either a year year in the proleptic Gregorian calendar that has a Thursday as its first day (January 1st), or a year year in the proleptic Gregorian calendar that has a Wednesday as its first day (January 1st) and where year is a number divisible by 400, or a number divisible by 4 but not by 100. All other week-years have 52 weeks.

The week number of the last day of a week-year with 53 weeks is 53; the week number of the last day of a week-year with 52 weeks is 52.

The week-year number of a particular day can be different than the number of the year that contains that day in the proleptic Gregorian calendar. The first week in a week-year y is the week that contains the first Thursday of the Gregorian year y.

For modern purposes, a week as defined here is equivalent to ISO weeks as defined in ISO 8601. [ISO8601]

A string is a valid week string representing a week-year year and week week if it consists of the following components in the given order:

1. Four or more ASCII digits, representing year, where year > 0
2. A U+002D HYPHEN-MINUS character (-)
3. A U+0057 LATIN CAPITAL LETTER W character (W)
4. Two ASCII digits, representing the week week, in the range 1 ≤ week ≤ maxweek, where maxweek is the week number of the last day of week-year year

The rules to parse a week string are as follows. This will return either a week-year number and week number, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Collect a sequence of characters that are ASCII digits. If the collected sequence is not at least four characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the year.

4. If year is not a number greater than zero, then fail.

5. If position is beyond the end of input or if the character at position is not a U+002D HYPHEN-MINUS character, then fail. Otherwise, move position forwards one character.

6. If position is beyond the end of input or if the character at position is not a U+0057 LATIN CAPITAL LETTER W character (W), then fail. Otherwise, move position forwards one character.

7. Collect a sequence of characters that are ASCII digits. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the week.

8. Let maxweek be the week number of the last day of year year.

9. If week is not a number in the range 1 ≤ week ≤ maxweek, then fail.

10. If position is not beyond the end of input, then fail.

11. Return the week-year number year and the week number week.

##### 2.4.5.9 Durations

A duration consists of a number of seconds.

Since months and seconds are not comparable (a month is not a precise number of seconds, but is instead a period whose exact length depends on the precise day from which it is measured) a duration as defined in this specification cannot include months (or years, which are equivalent to twelve months). Only durations that describe a specific number of seconds can be described.

A string is a valid duration string representing a duration t if it consists of either of the following:

- A literal U+0050 LATIN CAPITAL LETTER P character followed by one or more of the following subcomponents, in the order given, where the number of days, hours, minutes, and seconds corresponds to the same number of seconds as in t:

  1. One or more ASCII digits followed by a U+0044 LATIN CAPITAL LETTER D character, representing a number of days.

  2. A U+0054 LATIN CAPITAL LETTER T character followed by one or more of the following subcomponents, in the order given:

     1. One or more ASCII digits followed by a U+0048 LATIN CAPITAL LETTER H character, representing a number of hours.

     2. One or more ASCII digits followed by a U+004D LATIN CAPITAL LETTER M character, representing a number of minutes.

     3. The following components:

        1. One or more ASCII digits, representing a number of seconds.

        2. Optionally, a U+002E FULL STOP character (.) followed by one, two, or three ASCII digits, representing a fraction of a second.

        3. A U+0053 LATIN CAPITAL LETTER S character.

  This, as with a number of other date- and time-related microsyntaxes defined in this specification, is based on one of the formats defined in ISO 8601. [ISO8601]

- One or more duration time components, each with a different duration time component scale, in any order; the sum of the represented seconds being equal to the number of seconds in t.

  A duration time component is a string consisting of the following components:

  1. Zero or more space characters.

  2. One or more ASCII digits, representing a number of time units, scaled by the duration time component scale specified (see below) to represent a number of seconds.

  3. If the duration time component scale specified is 1 (i.e. the units are seconds), then, optionally, a U+002E FULL STOP character (.) followed by one, two, or three ASCII digits, representing a fraction of a second.

  4. Zero or more space characters.

  5. One of the following characters, representing the duration time component scale of the time unit used in the numeric part of the duration time component:

     U+0057 LATIN CAPITAL LETTER W character
     U+0077 LATIN SMALL LETTER W character
       Weeks. The scale is 604800.
     U+0044 LATIN CAPITAL LETTER D character
     U+0064 LATIN SMALL LETTER D character
       Days. The scale is 86400.
     U+0048 LATIN CAPITAL LETTER H character
     U+0068 LATIN SMALL LETTER H character
       Hours. The scale is 3600.
     U+004D LATIN CAPITAL LETTER M character
     U+006D LATIN SMALL LETTER M character
       Minutes. The scale is 60.
     U+0053 LATIN CAPITAL LETTER S character
     U+0073 LATIN SMALL LETTER S character
       Seconds. The scale is 1.

  6. Zero or more space characters.

  This is not based on any of the formats in ISO 8601. It is intended to be a more human-readable alternative to the ISO 8601 duration format.

The rules to parse a duration string are as follows. This will return either a duration or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Let months, seconds, and component count all be zero.

4. Let M-disambiguator be minutes.

   This flag's other value is months. It is used to disambiguate the "M" unit in ISO8601 durations, which use the same unit for months and minutes. Months are not allowed, but are parsed for future compatibility and to avoid misinterpreting ISO8601 durations that would be valid in other contexts.

5. Skip whitespace.

6. If position is past the end of input, then fail.

7. If the character in input pointed to by position is a U+0050 LATIN CAPITAL LETTER P character, then advance position to the next character, set M-disambiguator to months, and skip whitespace.

8. Run the following substeps in a loop, until a step requiring the loop to be broken or the entire algorithm to fail is reached:

   1. Let units be undefined. It will be assigned one of the following values: years, months, weeks, days, hours, minutes, and seconds.

   2. Let next character be undefined. It is used to process characters from the input.

   3. If position is past the end of input, then break the loop.

   4. If the character in input pointed to by position is a U+0054 LATIN CAPITAL LETTER T character, then advance position to the next character, set M-disambiguator to minutes, skip whitespace, and return to the top of the loop.

   5. Set next character to the character in input pointed to by position.

   6. If next character is a U+002E FULL STOP character (.), then let N equal zero. (Do not advance position. That is taken care of below.)

      Otherwise, if next character is an ASCII digit, then collect a sequence of characters that are ASCII digits, interpret the resulting sequence as a base-ten integer, and let N be that number.

      Otherwise next character is not part of a number; fail.

   7. If position is past the end of input, then fail.

   8. Set next character to the character in input pointed to by position, and this time advance position to the next character. (If next character was a U+002E FULL STOP character (.) before, it will still be that character this time.)

   9. If next character is a U+002E FULL STOP character (.), then run these substeps:

      1. Collect a sequence of characters that are ASCII digits. Let s be the resulting sequence.

      2. If s is the empty string, then fail.

      3. Let length be the number of characters in s.

      4. Let fraction be the result of interpreting s as a base-ten integer, and then dividing that number by 10length.

      5. Increment N by fraction.

      6. Skip whitespace.

      7. If position is past the end of input, then fail.

      8. Set next character to the character in input pointed to by position, and advance position to the next character.

      9. If next character is neither a U+0053 LATIN CAPITAL LETTER S character nor a U+0073 LATIN SMALL LETTER S character, then fail.

      10. Set units to seconds.

      Otherwise, run these substeps:

      1. If next character is a space character, then skip whitespace, set next character to the character in input pointed to by position, and advance position to the next character.

      2. If next character is a U+0059 LATIN CAPITAL LETTER Y character, or a U+0079 LATIN SMALL LETTER Y character, set units to years and set M-disambiguator to months.

         If next character is a U+004D LATIN CAPITAL LETTER M character or a U+006D LATIN SMALL LETTER M character, and M-disambiguator is months, then set units to months.

         If next character is a U+0057 LATIN CAPITAL LETTER W character or a U+0077 LATIN SMALL LETTER W character, set units to weeks and set M-disambiguator to minutes.

         If next character is a U+0044 LATIN CAPITAL LETTER D character or a U+0064 LATIN SMALL LETTER D character, set units to days and set M-disambiguator to minutes.

         If next character is a U+0048 LATIN CAPITAL LETTER H character or a U+0068 LATIN SMALL LETTER H character, set units to hours and set M-disambiguator to minutes.

         If next character is a U+004D LATIN CAPITAL LETTER M character or a U+006D LATIN SMALL LETTER M character, and M-disambiguator is minutes, then set units to minutes.

         If next character is a U+0053 LATIN CAPITAL LETTER S character or a U+0073 LATIN SMALL LETTER S character, set units to seconds and set M-disambiguator to minutes.

         Otherwise if next character is none of the above characters, then fail.

   10. Increment component count.

   11. Let multiplier be 1.

   12. If units is years, multiply multiplier by 12 and set units to months.

   13. If units is months, add the product of N and multiplier to months.

       Otherwise, run these substeps:

       1. If units is weeks, multiply multiplier by 7 and set units to days.

       2. If units is days, multiply multiplier by 24 and set units to hours.

       3. If units is hours, multiply multiplier by 60 and set units to minutes.

       4. If units is minutes, multiply multiplier by 60 and set units to seconds.

       5. Forcibly, units is now seconds. Add the product of N and multiplier to seconds.

   14. Skip whitespace.

9. If component count is zero, fail.

10. If months is not zero, fail.

11. Return the duration consisting of seconds seconds.

##### 2.4.5.10 Vaguer moments in time

A string is a valid date string with optional time if it is also one of the following:

- A valid date string
- A valid global date and time string

The rules to parse a date or time string are as follows. The algorithm will return either a date, a time, a global date and time, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Set start position to the same position as position.

4. Set the date present and time present flags to true.

5. Parse a date component to obtain year, month, and day. If this fails, then set the date present flag to false.

6. If date present is true, and position is not beyond the end of input, and the character at position is either a U+0054 LATIN CAPITAL LETTER T character (T) or a U+0020 SPACE character, then advance position to the next character in input.

   Otherwise, if date present is true, and either position is beyond the end of input or the character at position is neither a U+0054 LATIN CAPITAL LETTER T character (T) nor a U+0020 SPACE character, then set time present to false.

   Otherwise, if date present is false, set position back to the same position as start position.

7. If the time present flag is true, then parse a time component to obtain hour, minute, and second. If this returns nothing, then fail.

8. If the date present and time present flags are both true, but position is beyond the end of input, then fail.

9. If the date present and time present flags are both true, parse a time-zone offset component to obtain timezonehours and timezoneminutes. If this returns nothing, then fail.

10. If position is not beyond the end of input, then fail.

11. If the date present flag is true and the time present flag is false, then let date be the date with year year, month month, and day day, and return date.

    Otherwise, if the time present flag is true and the date present flag is false, then let time be the time with hour hour, minute minute, and second second, and return time.

    Otherwise, let time be the moment in time at year year, month month, day day, hours hour, minute minute, second second, subtracting timezonehours hours and timezoneminutes minutes, that moment in time being a moment in the UTC time zone; let timezone be timezonehours hours and timezoneminutes minutes from UTC; and return time and timezone.

#### 2.4.6 Colours

A simple colour consists of three 8-bit numbers in the range 0..255, representing the red, green, and blue components of the colour respectively, in the sRGB colour space. [SRGB]

A string is a valid simple colour if it is exactly seven characters long, and the first character is a U+0023 NUMBER SIGN character (#), and the remaining six characters are all ASCII hex digits, with the first two digits representing the red component, the middle two digits representing the green component, and the last two digits representing the blue component, in hexadecimal.

A string is a valid lowercase simple colour if it is a valid simple colour and doesn't use any characters in the range U+0041 LATIN CAPITAL LETTER A to U+0046 LATIN CAPITAL LETTER F.

The rules for parsing simple colour values are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either a simple colour or an error.

1. Let input be the string being parsed.

2. If input is not exactly seven characters long, then return an error.

3. If the first character in input is not a U+0023 NUMBER SIGN character (#), then return an error.

4. If the last six characters of input are not all ASCII hex digits, then return an error.

5. Let result be a simple colour.

6. Interpret the second and third characters as a hexadecimal number and let the result be the red component of result.

7. Interpret the fourth and fifth characters as a hexadecimal number and let the result be the green component of result.

8. Interpret the sixth and seventh characters as a hexadecimal number and let the result be the blue component of result.

9. Return result.

The rules for serialising simple colour values given a simple colour are as given in the following algorithm:

1. Let result be a string consisting of a single U+0023 NUMBER SIGN character (#).

2. Convert the red, green, and blue components in turn to two-digit hexadecimal numbers using lowercase ASCII hex digits, zero-padding if necessary, and append these numbers to result, in the order red, green, blue.

3. Return result, which will be a valid lowercase simple colour.

Some obsolete legacy attributes parse colours in a more complicated manner, using the rules for parsing a legacy colour value, which are given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either a simple colour or an error.

1. Let input be the string being parsed.

2. If input is the empty string, then return an error.

3. Strip leading and trailing whitespace from input.

4. If input is an ASCII case-insensitive match for the string "`transparent`", then return an error.

5. If input is an ASCII case-insensitive match for one of the named colours, then return the simple colour corresponding to that keyword. [CSSCOLOR]

   CSS2 System Colors are not recognised.

6. If input is four characters long, and the first character in input is a U+0023 NUMBER SIGN character (#), and the last three characters of input are all ASCII hex digits, then run these substeps:

   1. Let result be a simple colour.

   2. Interpret the second character of input as a hexadecimal digit; let the red component of result be the resulting number multiplied by 17.

   3. Interpret the third character of input as a hexadecimal digit; let the green component of result be the resulting number multiplied by 17.

   4. Interpret the fourth character of input as a hexadecimal digit; let the blue component of result be the resulting number multiplied by 17.

   5. Return result.

7. Replace any characters in input that have a Unicode code point greater than U+FFFF (i.e. any characters that are not in the basic multilingual plane) with the two-character string "`00`".

8. If input is longer than 128 characters, truncate input, leaving only the first 128 characters.

9. If the first character in input is a U+0023 NUMBER SIGN character (#), remove it.

10. Replace any character in input that is not an ASCII hex digit with the character U+0030 DIGIT ZERO (0).

11. While input's length is zero or not a multiple of three, append a U+0030 DIGIT ZERO (0) character to input.

12. Split input into three strings of equal length, to obtain three components. Let length be the length of those components (one third the length of input).

13. If length is greater than 8, then remove the leading length-8 characters in each component, and let length be 8.

14. While length is greater than two and the first character in each component is a U+0030 DIGIT ZERO (0) character, remove that character and reduce length by one.

15. If length is still greater than two, truncate each component, leaving only the first two characters in each.

16. Let result be a simple colour.

17. Interpret the first component as a hexadecimal number; let the red component of result be the resulting number.

18. Interpret the second component as a hexadecimal number; let the green component of result be the resulting number.

19. Interpret the third component as a hexadecimal number; let the blue component of result be the resulting number.

20. Return result.

The 2D graphics context has a separate colour syntax that also handles opacity.

#### 2.4.7 Space-separated tokens

A set of space-separated tokens is a string containing zero or more words (known as tokens) separated by one or more space characters, where words consist of any string of one or more characters, none of which are space characters.

A string containing a set of space-separated tokens may have leading or trailing space characters.

An unordered set of unique space-separated tokens is a set of space-separated tokens where none of the tokens are duplicated.

An ordered set of unique space-separated tokens is a set of space-separated tokens where none of the tokens are duplicated but where the order of the tokens is meaningful.

Sets of space-separated tokens sometimes have a defined set of allowed values. When a set of allowed values is defined, the tokens must all be from that list of allowed values; other values are non-conforming. If no such set of allowed values is provided, then all values are conforming.

How tokens in a set of space-separated tokens are to be compared (e.g. case-sensitively or not) is defined on a per-set basis.

When a user agent has to split a string on spaces, it must use the following algorithm:

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Let tokens be an ordered list of tokens, initially empty.

4. Skip whitespace

5. While position is not past the end of input:

   1. Collect a sequence of characters that are not space characters.

   2. Append the string collected in the previous step to tokens.

   3. Skip whitespace

6. Return tokens.

#### 2.4.8 Comma-separated tokens

A set of comma-separated tokens is a string containing zero or more tokens each separated from the next by a single U+002C COMMA character (,), where tokens consist of any string of zero or more characters, neither beginning nor ending with space characters, nor containing any U+002C COMMA characters (,), and optionally surrounded by space characters.

For instance, the string "`a ,b,,d d` " consists of four tokens: "a", "b", the empty string, and "d d". Leading and trailing whitespace around each token doesn't count as part of the token, and the empty string can be a token.

Sets of comma-separated tokens sometimes have further restrictions on what consists a valid token. When such restrictions are defined, the tokens must all fit within those restrictions; other values are non-conforming. If no such restrictions are specified, then all values are conforming.

When a user agent has to split a string on commas, it must use the following algorithm:

1. Let input be the string being parsed.

2. Let position be a pointer into input, initially pointing at the start of the string.

3. Let tokens be an ordered list of tokens, initially empty.

4. Token: If position is past the end of input, jump to the last step.

5. Collect a sequence of characters that are not U+002C COMMA characters (,). Let s be the resulting sequence (which might be the empty string).

6. Strip leading and trailing whitespace from s.

7. Append s to tokens.

8. If position is not past the end of input, then the character at position is a U+002C COMMA character (,); advance position past that character.

9. Jump back to the step labeled token.

10. Return tokens.

#### 2.4.9 References

A valid hash-name reference to an element of type type is a string consisting of a U+0023 NUMBER SIGN character (#) followed by a string which exactly matches the value of the `name` attribute of an element with type type in the document.

The rules for parsing a hash-name reference to an element of type type, given a context node scope, are as follows:

1. If the string being parsed does not contain a U+0023 NUMBER SIGN character, or if the first such character in the string is the last character in the string, then return null and abort these steps.

2. Let s be the string from the character immediately after the first U+0023 NUMBER SIGN character in the string being parsed up to the end of that string.

3. Return the first element of type type in tree order in the subtree rooted at scope that has an `id` attribute whose value is a case-sensitive match for s or a `name` attribute whose value is a compatibility caseless match for s.

#### 2.4.10 Media queries

A string is a valid media query list if it matches the `<media-query-list>` production of the Media Queries specification. [MQ]

A string matches the environment of the user if it is the empty string, a string consisting of only space characters, or is a media query list that matches the user's environment according to the definitions given in the Media Queries specification. [MQ]

### 2.5 URLs

#### 2.5.1 Terminology

A URL is a valid URL if it conforms to the authoring conformance requirements in the WHATWG URL standard. [URL]

A string is a valid non-empty URL if it is a valid URL but it is not the empty string.

A string is a valid URL potentially surrounded by spaces if, after stripping leading and trailing whitespace from it, it is a valid URL.

A string is a valid non-empty URL potentially surrounded by spaces if, after stripping leading and trailing whitespace from it, it is a valid non-empty URL.

This specification defines the URL `about:legacy-compat` as a reserved, though unresolvable, `about:` URL, for use in DOCTYPEs in HTML documents when needed for compatibility with XML tools. [ABOUT]

This specification defines the URL `about:srcdoc` as a reserved, though unresolvable, `about:` URL, that is used as the document's address of `iframe` `srcdoc` documents. [ABOUT]

The fallback base URL of a `Document` object is the absolute URL obtained by running these substeps:

1. If the `Document` is an `iframe` `srcdoc` document, then return the document base URL of the `Document`'s browsing context's browsing context container's node document and abort these steps.

2. If the document's address is `about:blank`, and the `Document`'s browsing context has a creator browsing context, then return the document base URL of the creator `Document`, and abort these steps.

3. Return the document's address.

The document base URL of a `Document` object is the absolute URL obtained by running these substeps:

1. If there is no `base` element that has an `href` attribute in the `Document`, then the document base URL is the `Document`'s fallback base URL; abort these steps.

2. Otherwise, the document base URL is the frozen base URL of the first `base` element in the `Document` that has an `href` attribute, in tree order.

#### 2.5.2 Resolving URLs

Resolving a URL is the process of taking a relative URL and obtaining the absolute URL that it implies.

To resolve a URL to an absolute URL relative to either another absolute URL or an element, the user agent must use the following steps. Resolving a URL can result in an error, in which case the URL is not resolvable.

1. Let url be the URL being resolved.

2. Let encoding be determined as follows:

   If the URL had a character encoding defined when the URL was created or defined or when this algorithm was invoked
     The URL character encoding is as defined.
   If the URL came from a script (e.g. as an argument to a method)
     The URL character encoding is the API URL character encoding specified by the script's settings object.
   If the URL came from a DOM node (e.g. from an element)
     The node has a `Document`, and the URL character encoding is the document's character encoding.

3. If encoding is a UTF-16 encoding, then change the value of encoding to UTF-8.

4. If the algorithm was invoked with an absolute URL to use as the base URL, let base be that absolute URL.

   Otherwise, let base be the element's base URL.

5. Apply the URL parser to url, with base as the base URL, with encoding as the encoding.

6. If this returns failure, then abort these steps with an error.

7. Let parsed URL be the result of the URL parser.

8. Let serialised URL be the result of apply the URL serialiser to parsed URL.

9. Return serialised URL as the resulting absolute URL and parsed URL as the resulting parsed URL.

Given an element, the element's base URL is the base URI of the element, as defined by the XML Base specification, with the base URI of the document entity being defined as the document base URL of the `Document` that owns the element. [XMLBASE]

For the purposes of the XML Base specification, user agents must act as if all `Document` objects represented XML documents.

It is possible for `xml:base` attributes to be present even in HTML fragments, as such attributes can be added dynamically using script. (Such scripts would not be conforming, however, as `xml:base` attributes are not allowed in HTML documents.)

#### 2.5.3 Dynamic changes to base URLs

When an `xml:base` attribute is set, changed, or removed, the attribute's element, and all descendant elements, are affected by a base URL change.

When a document's document base URL changes, all elements in that document are affected by a base URL change.

The following are base URL change steps, which run when an element is affected by a base URL change (as defined by the DOM specification):

If the element creates a hyperlink

  If the absolute URL identified by the hyperlink is being shown to the user, or if any data derived from that URL is affecting the display, then the `href` attribute should be re-resolved relative to the element and the UI updated appropriately.

  For example, the CSS `:link`/`:visited` pseudo-classes might have been affected.

  If the hyperlink has a `ping` attribute and its absolute URL(s) are being shown to the user, then the `ping` attribute's tokens should be re-resolved relative to the element and the UI updated appropriately.

If the element is a `q`, `blockquote`, `ins`, or `del` element with a `cite` attribute

  If the absolute URL identified by the `cite` attribute is being shown to the user, or if any data derived from that URL is affecting the display, then the URL should be re-resolved relative to the element and the UI updated appropriately.

Otherwise

  The element is not directly affected.

  For instance, changing the base URL doesn't affect the image displayed by `img` elements, although subsequent accesses of the `src` IDL attribute from script will return a new absolute URL that might no longer correspond to the image being shown.

### 2.6 Fetching resources

#### 2.6.1 Terminology

User agents can implement a variety of transfer protocols, but this specification mostly defines behaviour in terms of HTTP. [HTTP]

The HTTP GET method is equivalent to the default retrieval action of the protocol. For example, RETR in FTP. Such actions are idempotent and safe, in HTTP terms.

The HTTP response codes are equivalent to statuses in other protocols that have the same basic meanings. For example, a "file not found" error is equivalent to a 404 code, a server error is equivalent to a 5xx code, and so on.

The HTTP headers are equivalent to fields in other protocols that have the same basic meaning. For example, the HTTP authentication headers are equivalent to the authentication aspects of the FTP protocol.

#### 2.6.2 Processing model

When a user agent is to fetch a resource or URL, optionally from an origin origin, optionally using a specific `Document` or a specific URL as an override referrer source, and optionally with any of a blocking flag, a manual redirect flag, a force same-origin flag, and a block cookies flag, the following steps must be run. (When a URL is to be fetched, the URL identifies a resource to be obtained.)

1. If there is a specific override referrer source, and it is a URL, then let referrer be the override referrer source, and jump to the step labeled clean referrer.

2. Let document be the appropriate `Document` as given by the following list:

   If there is a specific override referrer source (which might be null)
     The override referrer source.
   When navigating
     The active document of the source browsing context.
   When fetching resources for an element
     The element's node document.

3. While document is an `iframe` `srcdoc` document, let document be document's browsing context's browsing context container's node document instead.

4. If document is null, or if the origin of document is not a scheme/host/port tuple, then set referrer to the empty string and jump to the step labeled cleaned referrer.

5. Let referrer be the document's address of document.

6. Clean referrer: Apply the URL parser to referrer and let parsed referrer be the resulting parsed URL.

7. Let referrer be the result of applying the URL serialiser to parsed referrer, with the exclude fragment flag set.

8. Cleaned referrer: If referrer is not the empty string, is not a `data:` URL, and is not the URL "`about:blank`", then generate the address of the resource from which Request-URIs are obtained as required by HTTP for the `Referer` (sic) header from referrer. [HTTP]

   Otherwise, the `Referer` (sic) header must be omitted, regardless of its value.

9. If the algorithm was not invoked with the blocking flag, perform the remaining steps in parallel.

10. If the `Document` with which any tasks queued by this algorithm would be associated doesn't have an associated browsing context, then abort these steps.

11. This is the main step.

    If the resource is to be obtained from an application cache, then use the data from that application cache, as if it had been obtained in the manner appropriate given its URL.

    If the resource is identified by an absolute URL, and the resource is to be obtained using an idempotent action (such as an HTTP GET or equivalent), and it is already being downloaded for other reasons (e.g. another invocation of this algorithm), and this request would be identical to the previous one (e.g. same `Accept` and `Origin` headers), and the user agent is configured such that it is to reuse the data from the existing download instead of initiating a new one, then use the results of the existing download instead of starting a new one.

    Otherwise, if the resource is identified by an absolute URL with a scheme that does not define a mechanism to obtain the resource (e.g. it is a `mailto:` URL) or that the user agent does not support, then act as if the resource was an HTTP 204 No Content response with no other metadata.

    Otherwise, if the resource is identified by the URL `about:blank`, then the resource is immediately available and consists of the empty string, with no metadata.

    Otherwise, at a time convenient to the user and the user agent, download (or otherwise obtain) the resource, applying the semantics of the relevant specifications (e.g. performing an HTTP GET or POST operation, or reading the file from disk, or expanding `data:` URLs, etc).

    For the purposes of the `Referer` (sic) header, use the address of the resource from which Request-URIs are obtained generated in the earlier step.

    For the purposes of the `Origin` header, if the fetching algorithm was explicitly initiated from an origin, then the origin that initiated the HTTP request is origin. Otherwise, this is a request from a "privacy-sensitive" context. [ORIGIN]

12. If the algorithm was not invoked with the block cookies flag, and there are cookies to be set, then the user agent must run the following substeps:

    1. Wait until ownership of the storage mutex can be taken by this instance of the fetching algorithm.

    2. Take ownership of the storage mutex.

    3. Update the cookies. [COOKIES]

    4. Release the storage mutex so that it is once again free.

13. If the fetched resource is an HTTP redirect or equivalent, then:

    If the force same-origin flag is set and the URL of the target of the redirect does not have the same origin as the URL for which the fetch algorithm was invoked

      Abort these steps and return failure from this algorithm, as if the remote host could not be contacted.

    If the manual redirect flag is set

      Continue, using the fetched resource (the redirect) as the result of the algorithm. If the calling algorithm subsequently requires the user agent to transparently follow the redirect, then the user agent must resume this algorithm from the main step, but using the target of the redirect as the resource to fetch, rather than the original resource.

    Otherwise

      First, apply any relevant requirements for redirects (such as showing any appropriate prompts). Then, redo main step, but using the target of the redirect as the resource to fetch, rather than the original resource. For HTTP requests, the new request must include the same headers as the original request, except for headers for which other requirements are specified (such as the `Host` header). [HTTP]

      The HTTP specification requires that 301, 302, and 307 redirects, when applied to methods other than the safe methods, not be followed without user confirmation. That would be an appropriate prompt for the purposes of the requirement in the paragraph above. [HTTP]

14. If the algorithm was not invoked with the blocking flag: When the resource is available, or if there is an error of some description, queue a task that uses the resource as appropriate. If the resource can be processed incrementally, as, for instance, with a progressively interlaced JPEG or an HTML file, additional tasks may be queued to process the data as it is downloaded. The task source for these tasks is the networking task source.

    Otherwise, return the resource or error information to the calling algorithm.

If the user agent can determine the actual length of the resource being fetched for an instance of this algorithm, and if that length is finite, then that length is the file's size. Otherwise, the subject of the algorithm (that is, the resource being fetched) has no known size. (For example, the HTTP `Content-Length` header might provide this information.)

The user agent must also keep track of the number of bytes downloaded for each instance of this algorithm. This number must exclude any out-of-band metadata, such as HTTP headers.

The application cache processing model introduces some changes to the networking model to handle the returning of cached resources.

The navigation processing model handles redirects itself, overriding the redirection handling that would be done by the fetching algorithm.

Whether the type sniffing rules apply to the fetched resource depends on the algorithm that invokes the rules — they are not always applicable.

#### 2.6.3 Encrypted HTTP and related security concerns

Anything in this specification that refers to HTTP also applies to HTTP-over-TLS, as represented by URLs using the `https:` scheme. [HTTP]

User agents should report certificate errors to the user and must either refuse to download resources sent with erroneous certificates or must act as if such resources were in fact served with no encryption.

User agents should warn the user that there is a potential problem whenever the user visits a page that the user has previously visited, if the page uses less secure encryption on the second visit.

Not doing so can result in users not noticing man-in-the-middle attacks.

If a user connects to a server with a self-signed certificate, the user agent could allow the connection but just act as if there had been no encryption. If the user agent instead allowed the user to override the problem and then displayed the page as if it was fully and safely encrypted, the user could be easily tricked into accepting man-in-the-middle connections.

If a user connects to a server with full encryption, but the page then refers to an external resource that has an expired certificate, then the user agent will act as if the resource was unavailable, possibly also reporting the problem to the user. If the user agent instead allowed the resource to be used, then an attacker could just look for "secure" sites that used resources from a different host and only apply man-in-the-middle attacks to that host, for example taking over scripts in the page.

If a user bookmarks a site that uses a CA-signed certificate, and then later revisits that site directly but the site has started using a self-signed certificate, the user agent could warn the user that a man-in-the-middle attack is likely underway, instead of simply acting as if the page was not encrypted.

#### 2.6.4 Determining the type of a resource

The Content-Type metadata of a resource must be obtained and interpreted in a manner consistent with the requirements of the MIME Sniffing specification. [MIMESNIFF]

The sniffed type of a resource must be found in a manner consistent with the requirements given in the MIME Sniffing specification for finding the sniffed media type of the relevant sequence of octets. [MIMESNIFF]

The rules for sniffing images specifically, the rules for distinguishing if a resource is text or binary, and the rules for sniffing audio and video specifically are also defined in the MIME Sniffing specification. These rules return a MIME type as their result. [MIMESNIFF]

It is imperative that the rules in the MIME Sniffing specification be followed exactly. When a user agent uses different heuristics for content type detection than the server expects, security problems can occur. For more details, see the MIME Sniffing specification. [MIMESNIFF]

#### 2.6.5 Extracting character encodings from `meta` elements

The algorithm for extracting a character encoding from a `meta` element, given a string s, is as follows. It either returns a character encoding or nothing.

1. Let position be a pointer into s, initially pointing at the start of the string.

2. Loop: Find the first seven characters in s after position that are an ASCII case-insensitive match for the word "`charset`". If no such match is found, return nothing and abort these steps.

3. Skip any space characters that immediately follow the word "`charset`" (there might not be any).

4. If the next character is not a U+003D EQUALS SIGN (=), then move position to point just before that next character, and jump back to the step labeled loop.

5. Skip any space characters that immediately follow the equals sign (there might not be any).

6. Process the next character as follows:

   If it is a U+0022 QUOTATION MARK character (") and there is a later U+0022 QUOTATION MARK character (") in s
   If it is a U+0027 APOSTROPHE character (') and there is a later U+0027 APOSTROPHE character (') in s
     Return the result of getting an encoding from the substring that is between this character and the next earliest occurrence of this character.
   If it is an unmatched U+0022 QUOTATION MARK character (")
   If it is an unmatched U+0027 APOSTROPHE character (')
   If there is no next character
     Return nothing.
   Otherwise
     Return the result of getting an encoding from the substring that consists of this character up to but not including the first space character or U+003B SEMICOLON character (;), or the end of s, whichever comes first.

This algorithm is distinct from those in the HTTP specification (for example, HTTP doesn't allow the use of single quotes and requires supporting a backslash-escape mechanism that is not supported by this algorithm). While the algorithm is used in contexts that, historically, were related to HTTP, the syntax as supported by implementations diverged some time ago. [HTTP]

#### 2.6.6 CORS settings attributes

A CORS settings attribute is an enumerated attribute. The following table lists the keywords and states for the attribute — the keywords in the left column map to the states in the cell in the second column on the same row as the keyword.

Keyword | State | Brief description
--- | --- | ---
`anonymous` | Anonymous | Cross-origin CORS requests for the element will have the omit credentials flag set.
`use-credentials` | Use Credentials | Cross-origin CORS requests for the element will not have the omit credentials flag set.

The empty string is also a valid keyword, and maps to the Anonymous state. The attribute's invalid value default is the Anonymous state. For the purposes of reflection, the canonical case for the Anonymous state is the `anonymous` keyword. The missing value default, used when the attribute is omitted, is the No CORS state.

#### 2.6.7 CORS-enabled fetch

When the user agent is required to perform a potentially CORS-enabled fetch of an absolute URL URL with a mode mode that is either "No CORS", "Anonymous", or "Use Credentials", optionally using a specific `Document` or specific URL referrer source, with an origin origin, and with a default origin behaviour default which is either "taint" or "fail", it must run the first applicable set of steps from the following list. The default origin behaviour is only used if mode is "No CORS". This algorithm wraps the fetch algorithm above, and labels the obtained resource as either CORS-same-origin or CORS-cross-origin, or blocks the resource entirely.

If the URL has the same origin as origin
If the URL is a `data:` URL
If the URL is `about:blank`

  Run these substeps:

  1. Fetch URL, using referrer source if one was specified, with the manual redirect flag set.

  2. Loop: Wait for the fetch algorithm to know if the result is a redirect or not.

  3. Follow the first appropriate steps from the following list:

     If the result of the fetch is a redirect, and the origin of the target URL of the redirect is not the same origin as origin

       Set URL to the target URL of the redirect and return to the top of the potentially CORS-enabled fetch algorithm (this time, one of the other branches below might be taken, based on the value of mode).

     If the result of the fetch is a redirect

       The origin of the target URL of the redirect is the same origin as origin.

       Transparently follow the redirect and jump to the step labeled loop above.

     Otherwise

       The resource is available, it is not a redirect, and its origin is the same origin as origin.

       The tasks from the fetch algorithm are queued normally, and for the purposes of the calling algorithm, the obtained resource is CORS-same-origin.

If mode is "No CORS" and default is taint

  The URL does not have the same origin as origin.

  Fetch URL, using referrer source if one was specified.

  The tasks from the fetch algorithm are queued normally, but for the purposes of the calling algorithm, the obtained resource is CORS-cross-origin. The user agent may report a cross-origin resource access failure to the user (e.g. in a debugging console).

If mode is "No CORS"

  The URL does not have the same origin as origin, and default is fail.

  Discard any data fetched as part of this algorithm, and prevent any tasks from such invocations of the fetch algorithm from being queued. For the purposes of the calling algorithm, the user agent must act as if there was a fatal network error and no resource was obtained. The user agent may report a cross-origin resource access failure to the user (e.g. in a debugging console).

If mode is "Anonymous" or "Use Credentials"

  The URL does not have the same origin as origin.

  Run these steps:

  1. Perform a cross-origin request with the request URL set to URL, with the CORS referrer source set to referrer source if one was specified, the source origin set to origin, and with the omit credentials flag set if mode is "Anonymous" and not set otherwise. [FETCH]

  2. Wait for the CORS cross-origin request status to have a value.

  3. Jump to the appropriate step from the following list:

     If the CORS cross-origin request status is not success

       Discard all fetched data and prevent any tasks from the fetch algorithm from being queued. For the purposes of the calling algorithm, the user agent must act as if there was a fatal network error and no resource was obtained. If a CORS resource sharing check failed, the user agent may report a cross-origin resource access failure to the user (e.g. in a debugging console).

     If the CORS cross-origin request status is success

       The tasks from the fetch algorithm are queued normally, and for the purposes of the calling algorithm, the obtained resource is CORS-same-origin.

### 2.7 Common DOM interfaces

#### 2.7.1 Reflecting content attributes in IDL attributes

Some IDL attributes are defined to reflect a particular content attribute. This means that on getting, the IDL attribute returns the current value of the content attribute, and on setting, the IDL attribute changes the value of the content attribute to the given value.

In general, on getting, if the content attribute is not present, the IDL attribute must act as if the content attribute's value is the empty string; and on setting, if the content attribute is not present, it must first be added.

If a reflecting IDL attribute is a `DOMString` attribute whose content attribute is defined to contain a URL, then on getting, the IDL attribute must resolve the value of the content attribute relative to the element and return the resulting absolute URL if that was successful, or the empty string otherwise; and on setting, must set the content attribute to the specified literal value. If the content attribute is absent, the IDL attribute must return the default value, if the content attribute has one, or else the empty string.

If a reflecting IDL attribute is a `DOMString` attribute whose content attribute is defined to contain one or more URLs, then on getting, the IDL attribute must split the content attribute on spaces and return the concatenation of resolving each token URL to an absolute URL relative to the element, with a single U+0020 SPACE character between each URL, ignoring any tokens that did not resolve successfully. If the content attribute is absent, the IDL attribute must return the default value, if the content attribute has one, or else the empty string. On setting, the IDL attribute must set the content attribute to the specified literal value.

If a reflecting IDL attribute is a `DOMString` attribute whose content attribute is an enumerated attribute, and the IDL attribute is limited to only known values, then, on getting, the IDL attribute must return the conforming value associated with the state the attribute is in (in its canonical case), if any, or the empty string if the attribute is in a state that has no associated keyword value or if the attribute is not in a defined state (e.g. the attribute is missing and there is no missing value default); and on setting, the content attribute must be set to the specified new value.

If a reflecting IDL attribute is a nullable `DOMString` attribute whose content attribute is an enumerated attribute, then, on getting, if the corresponding content attribute is in its missing value default then the IDL attribute must return null, otherwise, the IDL attribute must return the conforming value associated with the state the attribute is in (in its canonical case); and on setting, if the new value is null, the content attribute must be removed, and otherwise, the content attribute must be set to the specified new value.

If a reflecting IDL attribute is a `DOMString` attribute but doesn't fall into any of the above categories, then the getting and setting must be done in a transparent, case-preserving manner.

If a reflecting IDL attribute is a `boolean` attribute, then on getting the IDL attribute must return true if the content attribute is set, and false if it is absent. On setting, the content attribute must be removed if the IDL attribute is set to false, and must be set to the empty string if the IDL attribute is set to true. (This corresponds to the rules for boolean content attributes.)

If a reflecting IDL attribute has a signed integer type (`long`) then, on getting, the content attribute must be parsed according to the rules for parsing signed integers, and if that is successful, and the value is in the range of the IDL attribute's type, the resulting value must be returned. If, on the other hand, it fails or returns an out of range value, or if the attribute is absent, then the default value must be returned instead, or 0 if there is no default value. On setting, the given value must be converted to the shortest possible string representing the number as a valid integer and then that string must be used as the new content attribute value.

If a reflecting IDL attribute has a signed integer type (`long`) that is limited to only non-negative numbers then, on getting, the content attribute must be parsed according to the rules for parsing non-negative integers, and if that is successful, and the value is in the range of the IDL attribute's type, the resulting value must be returned. If, on the other hand, it fails or returns an out of range value, or if the attribute is absent, the default value must be returned instead, or −1 if there is no default value. On setting, if the value is negative, the user agent must throw an `IndexSizeError` exception. Otherwise, the given value must be converted to the shortest possible string representing the number as a valid non-negative integer and then that string must be used as the new content attribute value.

If a reflecting IDL attribute has an unsigned integer type (`unsigned long`) then, on getting, the content attribute must be parsed according to the rules for parsing non-negative integers, and if that is successful, and the value is in the range 0 to 2147483647 inclusive, the resulting value must be returned. If, on the other hand, it fails or returns an out of range value, or if the attribute is absent, the default value must be returned instead, or 0 if there is no default value. On setting, first, if the new value is in the range 0 to 2147483647, then let n be the new value, otherwise let n be the default value, or 0 if there is no default value; then, n must be converted to the shortest possible string representing the number as a valid non-negative integer and that string must be used as the new content attribute value.

If a reflecting IDL attribute has an unsigned integer type (`unsigned long`) that is limited to only non-negative numbers greater than zero, then the behaviour is similar to the previous case, but zero is not allowed. On getting, the content attribute must first be parsed according to the rules for parsing non-negative integers, and if that is successful, and the value is in the range 1 to 2147483647 inclusive, the resulting value must be returned. If, on the other hand, it fails or returns an out of range value, or if the attribute is absent, the default value must be returned instead, or 1 if there is no default value. On setting, if the value is zero, the user agent must throw an `IndexSizeError` exception. Otherwise, first, if the new value is in the range 1 to 2147483647, then let n be the new value, otherwise let n be the default value, or 1 if there is no default value; then, n must be converted to the shortest possible string representing the number as a valid non-negative integer and that string must be used as the new content attribute value.

If a reflecting IDL attribute has a floating-point number type (`double` or `unrestricted double`), then, on getting, the content attribute must be parsed according to the rules for parsing floating-point number values, and if that is successful, the resulting value must be returned. If, on the other hand, it fails, or if the attribute is absent, the default value must be returned instead, or 0.0 if there is no default value. On setting, the given value must be converted to the best representation of the number as a floating-point number and then that string must be used as the new content attribute value.

If a reflecting IDL attribute has a floating-point number type (`double` or `unrestricted double`) that is limited to numbers greater than zero, then the behaviour is similar to the previous case, but zero and negative values are not allowed. On getting, the content attribute must be parsed according to the rules for parsing floating-point number values, and if that is successful and the value is greater than 0.0, the resulting value must be returned. If, on the other hand, it fails or returns an out of range value, or if the attribute is absent, the default value must be returned instead, or 0.0 if there is no default value. On setting, if the value is less than or equal to zero, then the value must be ignored. Otherwise, the given value must be converted to the best representation of the number as a floating-point number and then that string must be used as the new content attribute value.

The values Infinity and Not-a-Number (NaN) values throw an exception on setting, as defined in the Web IDL specification. [WEBIDL]

If a reflecting IDL attribute has the type `DOMTokenList` or `DOMSettableTokenList`, then on getting it must return a `DOMTokenList` or `DOMSettableTokenList` object (as appropriate) whose associated element is the element in question and whose associated attribute's local name is the name of the attribute in question. The same `DOMTokenList` or `DOMSettableTokenList` object must be returned every time for each attribute.

If a reflecting IDL attribute has the type `HTMLElement`, or an interface that descends from `HTMLElement`, then, on getting, it must run the following algorithm (stopping at the first point where a value is returned):

1. If the corresponding content attribute is absent, then the IDL attribute must return null.
2. Let candidate be the element that the `document.getElementById()` method would find when called on the content attribute's element's node document if it were passed as its argument the current value of the corresponding content attribute.
3. If candidate is null, or if it is not type-compatible with the IDL attribute, then the IDL attribute must return null.
4. Otherwise, it must return candidate.

On setting, if the given element has an `id` attribute, and has the same home subtree as the element of the attribute being set, and the given element is the first element in that home subtree whose ID is the value of that `id` attribute, then the content attribute must be set to the value of that `id` attribute. Otherwise, the content attribute must be set to the empty string.

#### 2.7.2 Collections

The `HTMLAllCollection`, `HTMLFormControlsCollection`, `HTMLOptionsCollection`, and `HTMLPropertiesCollection` interfaces are collections derived from the `HTMLCollection` interface.

##### 2.7.2.1 The `HTMLAllCollection` interface

The `HTMLAllCollection` interface is used for the legacy `document.all` attribute. It operates similarly to `HTMLCollection`; the main differences are that its `namedItem()` method returns an `HTMLCollection` object when there are multiple matching elements, and that its `item()` method can be used as a synonym for its `namedItem()` method.

All `HTMLAllCollection` objects are rooted at a `Document` and have a filter that matches all elements, so the elements represented by the collection of an `HTMLAllCollection` object consist of all the descendant elements of the root `Document`.

    interface HTMLAllCollection : HTMLCollection {
      // inherits length and 'getter'
      Element? item(unsigned long index);
      (HTMLCollection or Element)? item(DOMString name);
      legacycaller getter (HTMLCollection or Element)? namedItem(DOMString name); // shadows inherited namedItem()
    };

collection . `length`

  Returns the number of elements in the collection.

element = collection . `item`(index)
collection[index]

  Returns the item with index index from the collection. The items are sorted in tree order.

element = collection . `item`(name)
collection = collection . `item`(name)
element = collection . `namedItem`(name)
collection = collection . `namedItem`(name)
element = collection(name)
collection = collection(name)
collection[name]

  Returns the item with ID or name name from the collection.

  If there are multiple matching items, then an `HTMLCollection` object containing all those elements is returned.

  Only `button`, `form`, `iframe`, `input`, `map`, `meta`, `object`, `select`, and `textarea` elements can have a name for the purpose of this method; their name is given by the value of their `name` attribute.

The object's supported property indices are as defined for `HTMLCollection` objects.

The `item()` method, when invoked with a numeric argument, must act like the item() method inherited from `HTMLCollection`.

The following elements are "all"-named elements: `a`, `applet`, `button`, `embed`, `form`, `frame`, `frameset`, `iframe`, `img`, `input`, `map`, `meta`, `object`, `select`, and `textarea`

The supported property names consist of the non-empty values of all the `id` attributes of all the elements represented by the collection, and the non-empty values of all the `name` attributes of all the "all"-named elements represented by the collection, in tree order, ignoring later duplicates, with the `id` of an element preceding its `name` if it contributes both, they differ from each other, and neither is the duplicate of an earlier entry.

The properties exposed in this way must be unenumerable.

The `item(name)` and `namedItem(name)` methods must act according to the following algorithm:

1. If name is the empty string, return null and stop the algorithm.

2. Let collection be an `HTMLCollection` object rooted at the same `Document` as the `HTMLAllCollection` object on which the method was invoked, whose filter matches only elements that are either:

   - "all"-named elements with a `name` attribute equal to name, or,
   - elements with an ID equal to name.
3. If, at the time the method is called, there is exactly one node in collection, then return that node and stop the algorithm.
4. Otherwise, if, at the time the method is called, collection is empty, return null and stop the algorithm.
5. Otherwise, return collection.

##### 2.7.2.2 The `HTMLFormControlsCollection` interface

The `HTMLFormControlsCollection` interface is used for collections of listed elements in `form` and `fieldset` elements.

    interface HTMLFormControlsCollection : HTMLCollection {
      // inherits length and item()
      legacycaller getter (RadioNodeList or Element)? namedItem(DOMString name); // shadows inherited namedItem()
    };

    interface RadioNodeList : NodeList {
      attribute DOMString value;
    };

collection . `length`

  Returns the number of elements in the collection.

element = collection . `item`(index)
collection[index]

  Returns the item with index index from the collection. The items are sorted in tree order.

element = collection . `namedItem`(name)
radioNodeList = collection . `namedItem`(name)
collection[name]
collection(name)

  Returns the item with ID or `name` name from the collection.

  If there are multiple matching items, then a `RadioNodeList` object containing all those elements is returned.

radioNodeList . value [ = value ]

  Returns the value of the first checked radio button represented by the object.

  Can be set, to check the first radio button with the given value represented by the object.

The object's supported property indices are as defined for `HTMLCollection` objects.

The supported property names consist of the non-empty values of all the `id` and `name` attributes of all the elements represented by the collection, in tree order, ignoring later duplicates, with the `id` of an element preceding its `name` if it contributes both, they differ from each other, and neither is the duplicate of an earlier entry.

The properties exposed in this way must be unenumerable.

The `namedItem(name)` method must act according to the following algorithm:

1. If name is the empty string, return null and stop the algorithm.
2. If, at the time the method is called, there is exactly one node in the collection that has either an `id` attribute or a `name` attribute equal to name, then return that node and stop the algorithm.
3. Otherwise, if there are no nodes in the collection that have either an `id` attribute or a `name` attribute equal to name, then return null and stop the algorithm.
4. Otherwise, create a new `RadioNodeList` object representing a live view of the `HTMLFormControlsCollection` object, further filtered so that the only nodes in the `RadioNodeList` object are those that have either an `id` attribute or a `name` attribute equal to name. The nodes in the `RadioNodeList` object must be sorted in tree order.
5. Return that `RadioNodeList` object.

Members of the `RadioNodeList` interface inherited from the `NodeList` interface must behave as they would on a `NodeList` object.

The `value` IDL attribute on the `RadioNodeList` object, on getting, must return the value returned by running the following steps:

1. Let element be the first element in tree order represented by the `RadioNodeList` object that is an `input` element whose `type` attribute is in the Radio Button state and whose checkedness is true. Otherwise, let it be null.

2. If element is null, return the empty string.

3. If element is an element with no `value` attribute, return the string "`on`".

4. Otherwise, return the value of element's `value` attribute.

On setting, the `value` IDL attribute must run the following steps:

1. If the new value is the string "`on`": let element be the first element in tree order represented by the `RadioNodeList` object that is an `input` element whose `type` attribute is in the Radio Button state and whose `value` content attribute is either absent, or present and equal to the new value, if any. If no such element exists, then instead let element be null.

   Otherwise: let element be the first element in tree order represented by the `RadioNodeList` object that is an `input` element whose `type` attribute is in the Radio Button state and whose `value` content attribute is present and equal to the new value, if any. If no such element exists, then instead let element be null.

2. If element is not null, then set its checkedness to true.

##### 2.7.2.3 The `HTMLOptionsCollection` interface

The `HTMLOptionsCollection` interface is used for collections of `option` elements. It is always rooted on a `select` element and has attributes and methods that manipulate that element's descendants.

    interface HTMLOptionsCollection : HTMLCollection {
      // inherits item()
      attribute unsigned long length; // shadows inherited length
      legacycaller HTMLOptionElement? (DOMString name);
      setter creator void (unsigned long index, HTMLOptionElement? option);
      void add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
      void remove(long index);
      attribute long selectedIndex;
    };

collection . `length` [ = value ]

  Returns the number of elements in the collection.

  When set to a smaller number, truncates the number of `option` elements in the corresponding container.

  When set to a greater number, adds new blank `option` elements to that container.

element = collection . `item`(index)
collection[index]

  Returns the item with index index from the collection. The items are sorted in tree order.

element = collection . `namedItem`(name)
nodeList = collection . `namedItem`(name)
collection[name]
collection(name)

  Returns the item with ID or `name` name from the collection.

  If there are multiple matching items, then the first is returned.

collection . `add`(element [, before ] )

  Inserts element before the node given by before.

  The before argument can be a number, in which case element is inserted before the item with that number, or an element from the collection, in which case element is inserted before that element.

  If before is omitted, null, or a number out of range, then element will be added at the end of the list.

  This method will throw a `HierarchyRequestError` exception if element is an ancestor of the element into which it is to be inserted.

collection . `selectedIndex` [ = value ]

  Returns the index of the first selected item, if any, or −1 if there is no selected item.

  Can be set, to change the selection.

The object's supported property indices are as defined for `HTMLCollection` objects.

On getting, the `length` attribute must return the number of nodes represented by the collection.

On setting, the behaviour depends on whether the new value is equal to, greater than, or less than the number of nodes represented by the collection at that time. If the number is the same, then setting the attribute must do nothing. If the new value is greater, then n new `option` elements with no attributes and no child nodes must be appended to the `select` element on which the `HTMLOptionsCollection` is rooted, where n is the difference between the two numbers (new value minus old value). Mutation events must be fired as if a `DocumentFragment` containing the new `option` elements had been inserted. If the new value is lower, then the last n nodes in the collection must be removed from their parent nodes, where n is the difference between the two numbers (old value minus new value).

Setting `length` never removes or adds any `optgroup` elements, and never adds new children to existing `optgroup` elements (though it can remove children from them).

The supported property names consist of the non-empty values of all the `id` and `name` attributes of all the elements represented by the collection, in tree order, ignoring later duplicates, with the `id` of an element preceding its `name` if it contributes both, they differ from each other, and neither is the duplicate of an earlier entry.

The properties exposed in this way must be unenumerable.

The legacy caller of the `HTMLOptionsCollection` interface must act like the `namedItem()` method on the ancestor `HTMLCollection` interface.

When the user agent is to set the value of a new indexed property or set the value of an existing indexed property for a given property index index to a new value value, it must run the following algorithm:

1. If value is null, invoke the steps for the `remove` method with index as the argument, and abort these steps.

2. Let length be the number of nodes represented by the collection.

3. Let n be index minus length.

4. If n is greater than zero, then append a `DocumentFragment` consisting of n-1 new `option` elements with no attributes and no child nodes to the `select` element on which the `HTMLOptionsCollection` is rooted.

5. If n is greater than or equal to zero, append value to the `select` element. Otherwise, replace the indexth element in the collection by value.

The `add(element, before)` method must act according to the following algorithm:

1. If element is an ancestor of the `select` element on which the `HTMLOptionsCollection` is rooted, then throw a `HierarchyRequestError` exception and abort these steps.

2. If before is an element, but that element isn't a descendant of the `select` element on which the `HTMLOptionsCollection` is rooted, then throw a `NotFoundError` exception and abort these steps.

3. If element and before are the same element, then return and abort these steps.

4. If before is a node, then let reference be that node. Otherwise, if before is an integer, and there is a beforeth node in the collection, let reference be that node. Otherwise, let reference be null.

5. If reference is not null, let parent be the parent node of reference. Otherwise, let parent be the `select` element on which the `HTMLOptionsCollection` is rooted.

6. Act as if the DOM `insertBefore()` method was invoked on the parent node, with element as the first argument and reference as the second argument.

The `remove(index)` method must act according to the following algorithm:

1. If the number of nodes represented by the collection is zero, abort these steps.

2. If index is not a number greater than or equal to 0 and less than the number of nodes represented by the collection, abort these steps.

3. Let element be the indexth element in the collection.

4. Remove element from its parent node.

The `selectedIndex` IDL attribute must act like the identically named attribute on the `select` element on which the `HTMLOptionsCollection` is rooted

##### 2.7.2.4 The `HTMLPropertiesCollection` interface

The `HTMLPropertiesCollection` interface is used for collections of elements that add name-value pairs to a particular item in the microdata model.

    interface HTMLPropertiesCollection : HTMLCollection {
      // inherits length and item()
      getter PropertyNodeList? namedItem(DOMString name); // shadows inherited namedItem()
      [SameObject] readonly attribute DOMString[] names;
    };

    typedef sequence<any> PropertyValueArray;

    interface PropertyNodeList : NodeList {
      PropertyValueArray getValues();
    };

collection . `length`

  Returns the number of elements in the collection.

element = collection . `item`(index)
collection[index]

  Returns the element with index index from the collection. The items are sorted in tree order.

propertyNodeList = collection . `namedItem`(name)

  Returns a `PropertyNodeList` object containing any elements that add a property named name.

collection[name]

  Returns a `PropertyNodeList` object containing any elements that add a property named name. The name index has to be one of the values listed in the `names` list.

collection . `names`

  Returns an array with the property names of the elements in the collection.

propertyNodeList . `getValues`()

  Returns an array of the various values that the relevant elements have.

The object's supported property indices are as defined for `HTMLCollection` objects.

The supported property names consist of the property names of all the elements represented by the collection, in tree order, ignoring later duplicates.

The properties exposed in this way must be unenumerable.

The `names` attribute must return a live read only array object giving the property names of all the elements represented by the collection, listed in tree order, but with duplicates removed, leaving only the first occurrence of each name.

The `namedItem(name)` method must return a `PropertyNodeList` object representing a live view of the `HTMLPropertiesCollection` object, further filtered so that the only nodes in the `PropertyNodeList` object are those that have a property name equal to name. The nodes in the `PropertyNodeList` object must be sorted in tree order, and the same object must be returned each time a particular name is queried from a given `HTMLPropertiesCollection` object.

Members of the `PropertyNodeList` interface inherited from the `NodeList` interface must behave as they would on a `NodeList` object.

The `getValues` method the `PropertyNodeList` object must return a newly constructed array whose values are the values obtained from the `itemValue` IDL attribute of each of the elements represented by the object, in tree order.

#### 2.7.3 The `DOMStringMap` interface

The `DOMStringMap` interface represents a set of name-value pairs. It exposes these using the scripting language's native mechanisms for property access.

When a `DOMStringMap` object is instantiated, it is associated with three algorithms, one for getting the list of name-value pairs, one for setting names to certain values, and one for deleting names.

    [OverrideBuiltins, Exposed=(Window,Worker)]
    interface DOMStringMap {
      getter DOMString (DOMString name);
      setter creator void (DOMString name, DOMString value);
      deleter void (DOMString name);
    };

The supported property names on a `DOMStringMap` object at any instant are the names of each pair returned from the algorithm for getting the list of name-value pairs at that instant, in the order returned.

To determine the value of a named property name in a `DOMStringMap`, the user agent must return the value component of the name-value pair whose name component is name in the list returned by the algorithm for getting the list of name-value pairs.

To set the value of a new or existing named property name to value value, the algorithm for setting names to certain values must be run, passing name as the name and the result of converting value to a `DOMString` as the value.

To delete an existing named property name, the algorithm for deleting names must be run, passing name as the name.

The `DOMStringMap` interface definition here is only intended for JavaScript environments. Other language bindings will need to define how `DOMStringMap` is to be implemented for those languages.

The `dataset` attribute on elements exposes the `data-*` attributes on the element.

Given the following fragment and elements with similar constructions:

    <img class="tower" id="tower5" data-x="12" data-y="5"
         data-ai="robotarget" data-hp="46" data-ability="flames"
         src="towers/rocket.png" alt="Rocket Tower">

...one could imagine a function `splashDamage()` that takes some arguments, the first of which is the element to process:

    function splashDamage(node, x, y, damage) {
      if (node.classList.contains('tower') && // checking the 'class' attribute
          node.dataset.x == x && // reading the 'data-x' attribute
          node.dataset.y == y) { // reading the 'data-y' attribute
        var hp = parseInt(node.dataset.hp); // reading the 'data-hp' attribute
        hp = hp - damage;
        if (hp < 0) {
          hp = 0;
          node.dataset.ai = 'dead'; // setting the 'data-ai' attribute
          delete node.dataset.ability; // removing the 'data-ability' attribute
        }
        node.dataset.hp = hp; // setting the 'data-hp' attribute
      }
    }

#### 2.7.4 DOMElementMap

The `DOMElementMap` interface represents a set of name-element mappings. It exposes these using the scripting language's native mechanisms for property access.

When a `DOMElementMap` object is instantiated, it is associated with three algorithms, one for getting the list of name-element mappings, one for mapping a name to a certain element, and one for deleting mappings by name.

    interface DOMElementMap {
      getter Element (DOMString name);
      setter creator void (DOMString name, Element value);
      deleter void (DOMString name);
    };

The supported property names on a `DOMElementMap` object at any instant are the names for each mapping returned from the algorithm for getting the list of name-element mappings at that instant, in the order returned.

To determine the value of a named property name in a `DOMElementMap`, the user agent must return the element component of the name-element mapping whose name component is name in the list returned by the algorithm for getting the list of name-element mappings.

To set the value of a new or existing named property name to value value, the algorithm for mapping a name to a certain element must be run, passing name as the name value as the element.

To delete an existing named property name, the algorithm for deleting mappings must be run, passing name as the name component of the mapping to be deleted.

The `DOMElementMap` interface definition here is only intended for JavaScript environments. Other language bindings will need to define how `DOMElementMap` is to be implemented for those languages.

#### 2.7.5 Transferable objects

Some objects support being copied and closed in one operation. This is called transferring the object, and is used in particular to transfer ownership of unsharable or expensive resources across worker boundaries.

The following `Transferable` types exist:

- `ArrayBuffer` [ECMA262]
- `CanvasProxy` (defined in this specification)
- `MessagePort` (defined in this specification)

The following IDL block formalizes this:

    typedef (ArrayBuffer or CanvasProxy or MessagePort) Transferable;

To transfer a `Transferable` object to a new owner, the user agent must run the steps defined for the type of object in question. The steps will return a new object of the same type, and will permanently neuter the original object. (This is an irreversible and non-idempotent operation; once an object has been transferred, it cannot be transferred, or indeed used, again.)

To transfer an `ArrayBuffer` object old to a new owner owner, a user agent must create a new `ArrayBuffer` object pointing at the same underlying data as old, thus obtaining new, must neuter the old object, and must finally return new. [ECMA262]

Rules for how to transfer a `CanvasProxy` object and how to transfer a `MessagePort` object are given in the relevant sections of this specification.

#### 2.7.6 Safe passing of structured data

When a user agent is required to obtain a structured clone of a value, optionally with a transfer map, it must run the following algorithm, which either returns a separate value, or throws an exception. If a transfer map is provided, it consists of an association list of `Transferable` objects to placeholder objects.

1. Let input be the value being cloned.

2. Let transfer map be the transfer map passed to the algorithm, if any, or the empty list otherwise.

3. Let memory be an association list of pairs of objects, initially empty. This is used to handle duplicate references. In each pair of objects, one is called the source object and the other the destination object.

4. For each mapping in transfer map, add a mapping from the `Transferable` object (the source object) to the placeholder object (the destination object) to memory.

5. Let output be the value resulting from calling the internal structured cloning algorithm with input as the "input" argument, and memory as the "memory" argument.

6. Return output.

The internal structured cloning algorithm is always called with two arguments, input and memory, and its behaviour is as follows:

1. If input is the source object of a pair of objects in memory, then return the destination object in that pair of objects and abort these steps.

2. If input is a primitive value, then return that value and abort these steps.

3. Let deep clone be none.

4. The input value is an object. Jump to the appropriate step below:

   If input is a `Boolean` object

     Let output be a newly constructed Boolean object with the same value as input.

   If input is a `Number` object

     Let output be a newly constructed Number object with the same value as input.

   If input is a `String` object

     Let output be a newly constructed String object with the same value as input.

   If input is a `Date` object

     Let output be a newly constructed `Date` object with the same value as input.

   If input is a `RegExp` object

     Let output be a newly constructed `RegExp` object with the same pattern and flags as input.

     The value of the `lastIndex` property is not copied.

   If input is a `Blob` object

     If input has been disabled through the `close()` method, throw a `DataCloneError` exception and abort the overall structured clone algorithm. Otherwise, let output be a newly constructed object of the same class as input, corresponding to the same underlying data.

   If input is a `FileList` object

     Let output be a newly constructed `FileList` object containing a list of newly constructed `File` objects corresponding to the same underlying data as those in input, maintaining their relative order.

   If input is an `ImageData` object

     Let output be a newly constructed `ImageData` object whose `width` and `height` have values equal to the corresponding attributes on input, and whose `data` attribute has the value obtained from invoking the internal structured cloning algorithm recursively with the value of the `data` attribute on input as the new "input" argument and memory as the new "memory" argument.

   If input is an `ImageBitmap` object

     Let output be a newly constructed `ImageBitmap` object whose bitmap data is a copy of input's bitmap data.

   If input is an `ArrayBuffer` object

     If input has been neutered, throw a `DataCloneError` exception and abort the overall structured clone algorithm. Otherwise, let output be a newly constructed `ArrayBuffer` object whose contents are a copy of input's contents, with the same length.

   If input is an object with a [[DataView]] internal slot

     Let output be a newly constructed object of the same class as input, with its [[DataView]] internal property present, its [[ViewedArrayBuffer]] internal property set to the value obtained from invoking the internal structured cloning algorithm recursively with the value of the internal property on input as the new "input" argument and memory as the new "memory" argument, and with the [[ByteLength]] and [[ByteOffset]] internal properties set to the same value as their counterparts on input.

   If input is an `Array` object

     Let output be a newly constructed empty `Array` object whose `length` is equal to the `length` of input, and set deep clone to own.

     This means that the length of sparse arrays is preserved.

   If input is an `Object` object

     Let output be a newly constructed empty `Object` object, and set deep clone to own.

   If input is a `Map` object

     Let output be a newly constructed empty `Map` object, and set deep clone to map.

   If input is a `Set` object

     Let output be a newly constructed empty `Set` object, and set deep clone to set.

   If input is an object that another specification defines how to clone

     Let output be a clone of the object as defined by the other specification.

   If input is another native object type (e.g. `Error`, `Function`)
   If input is a host object (e.g. a DOM node)

     Throw a `DataCloneError` exception and abort the overall structured clone algorithm.

   For the purposes of the algorithm above, an object is a particular type of object class if its [[Class]] internal property is equal to class.

   For example, "input is an `Object` object" if input's [[Class]] internal property is equal to the string "`Object`".

5. Add a mapping from input (the source object) to output (the destination object) to memory.

6. If deep clone is set to map, then run these substeps. These substeps use the terminology and typographic conventions used in the JavaScript specification's definition of Maps. [ECMA262]

   1. Let source be the List that is the value of input's [[MapData]] internal slot, if any. If there is no such slot, then instead throw a `DataCloneError` exception and abort the overall structured clone algorithm. [ECMA262]

   2. Let target be the List that is the value of output's [[MapData]] internal slot.

   3. For each Record {[[key]], [[value]]} entry that is an element of source, run the following substeps:

      1. Let key have the value obtained from invoking the internal structured cloning algorithm recursively with entry.[[key]] as the new "input" argument and memory as the new "memory" argument.

      2. Let value have the value obtained from invoking the internal structured cloning algorithm recursively with entry.[[value]] as the new "input" argument and memory as the new "memory" argument.

      3. Let new entry be the Record {[[key]]: key, [[value]]: value}.

      4. Append new entry as the last element of target.

   4. Set deep clone to own.

7. If deep clone is set to set, then run these substeps. These substeps use the terminology and typographic conventions used in the JavaScript specification's definition of Sets. [ECMA262]

   1. Let source be the List that is the value of input's [[SetData]] internal slot, if any. If there is no such slot, then instead throw a `DataCloneError` exception and abort the overall structured clone algorithm. [ECMA262]

   2. Let target be the List that is the value of output's [[SetData]] internal slot.

   3. For each entry that is an element of source that is not empty, run the following substeps:

      1. Let new entry have the value obtained from invoking the internal structured cloning algorithm recursively with entry as the new "input" argument and memory as the new "memory" argument.

      2. Append new entry as the last element of target.

   4. Set deep clone to own.

8. If deep clone is set to own, then, for each enumerable own property in input, run the following steps:

   1. Let name be the name of the property.

   2. Let source value be the result of calling the [[Get]] internal method of input with the argument name. If the [[Get]] internal method of a property involved executing script, and that script threw an uncaught exception, then abort the overall structured clone algorithm, with that exception being passed through to the caller.

   3. Let cloned value be the result of invoking the internal structured cloning algorithm recursively with source value as the "input" argument and memory as the "memory" argument. If this results in an exception, then abort the overall structured clone algorithm, with that exception being passed through to the caller.

   4. Add a new property to output having the name name, and having the value cloned value.

   The order of the properties in the input and output objects must be the same, and any properties whose [[Get]] internal method involves running script must be processed in that same order.

   This does not walk the prototype chain.

   Property descriptors, setters, getters, and analogous features are not copied in this process. For example, the property in the input could be marked as read-only, but in the output it would just have the default state (typically read-write, though that could depend on the scripting environment).

   Properties of Array objects are not treated any differently than those of other Objects. In particular, this means that non-index properties of arrays are copied as well.

9. Return output.

This algorithm preserves cycles and preserves the identity of duplicate objects in graphs.

#### 2.7.7 Callbacks

The following callback function type is used in various APIs that interact with `File` objects:

    callback FileCallback = void (File file);

#### 2.7.8 Garbage collection

There is an implied strong reference from any IDL attribute that returns a pre-existing object to that object.

For example, the `document.location` attribute means that there is a strong reference from a `Document` object to its `Location` object. Similarly, there is always a strong reference from a `Document` to any descendant nodes, and from any node to its owner `Document`.

### 2.8 Namespaces

The HTML namespace is: `http://www.w3.org/1999/xhtml`

The MathML namespace is: `http://www.w3.org/1998/Math/MathML`

The SVG namespace is: `http://www.w3.org/2000/svg`

The XLink namespace is: `http://www.w3.org/1999/xlink`

The XML namespace is: `http://www.w3.org/XML/1998/namespace`

The XMLNS namespace is: `http://www.w3.org/2000/xmlns/`

Data mining tools and other user agents that perform operations on content without running scripts, evaluating CSS or XPath expressions, or otherwise exposing the resulting DOM to arbitrary content, may "support namespaces" by just asserting that their DOM node analogues are in certain namespaces, without actually exposing the above strings.

In the HTML syntax, namespace prefixes and namespace declarations do not have the same effect as in XML. For instance, the colon has no special meaning in HTML element names.