## 13 The XHTML syntax

This section only describes the rules for XML resources. Rules for `text/html` resources are discussed in the section above entitled "The HTML syntax".

### 13.1 Writing XHTML documents

The syntax for using HTML with XML, whether in XHTML documents or embedded in other XML documents, is defined in the XML and Namespaces in XML specifications. [XML] [XMLNS]

This specification does not define any syntax-level requirements beyond those defined for XML proper.

XML documents may contain a `DOCTYPE` if desired, but this is not required to conform to this specification. This specification does not define a public or system identifier, nor provide a formal DTD.

According to the XML specification, XML processors are not guaranteed to process the external DTD subset referenced in the DOCTYPE. This means, for example, that using entity references for characters in XHTML documents is unsafe if they are defined in an external file (except for `&lt;`, `&gt;`, `&amp;`, `&quot;` and `&apos;`).

### 13.2 Parsing XHTML documents

This section describes the relationship between XML and the DOM, with a particular emphasis on how this interacts with HTML.

An XML parser, for the purposes of this specification, is a construct that follows the rules given in the XML specification to map a string of bytes or characters into a `Document` object.

At the time of writing, no such rules actually exist.

An XML parser is either associated with a `Document` object when it is created, or creates one implicitly.

This `Document` must then be populated with DOM nodes that represent the tree structure of the input passed to the parser, as defined by the XML specification, the Namespaces in XML specification, and the DOM specification. DOM mutation events must not fire for the operations that the XML parser performs on the `Document`'s tree, but the user agent must act as if elements and attributes were individually appended and set respectively so as to trigger rules in this specification regarding what happens when an element is inserted into a document or has its attributes set, and the DOM specification's requirements regarding mutation observers mean that mutation observers are fired (unlike mutation events). [XML] [XMLNS] [DOM] [DOMEVENTS]

Between the time an element's start tag is parsed and the time either the element's end tag is parsed or the parser detects a well-formedness error, the user agent must act as if the element was in a stack of open elements.

This is used, e.g. by the `object` element to avoid instantiating plugins before the `param` element children have been parsed.

This specification provides the following additional information that user agents should use when retrieving an external entity: the public identifiers given in the following list all correspond to the URL given by this link. (This URL is a DTD containing the entity declarations for the names listed in the named character references section.) [XML]

- `-//W3C//DTD XHTML 1.0 Transitional//EN`
- `-//W3C//DTD XHTML 1.1//EN`
- `-//W3C//DTD XHTML 1.0 Strict//EN`
- `-//W3C//DTD XHTML 1.0 Frameset//EN`
- `-//W3C//DTD XHTML Basic 1.0//EN`
- `-//W3C//DTD XHTML 1.1 plus MathML 2.0//EN`
- `-//W3C//DTD XHTML 1.1 plus MathML 2.0 plus SVG 1.1//EN`
- `-//W3C//DTD MathML 2.0//EN`
- `-//WAPFORUM//DTD XHTML Mobile 1.0//EN`

Furthermore, user agents should attempt to retrieve the above external entity's content when one of the above public identifiers is used, and should not attempt to retrieve any other external entity's content.

This is not strictly a violation of the XML specification, but it does contradict the spirit of the XML specification's requirements. This is motivated by a desire for user agents to all handle entities in an interoperable fashion without requiring any network access for handling external subsets. [XML]

XML parsers can be invoked with XML scripting support enabled or disabled. Except where otherwise specified, XML parsers are invoked with XML scripting support enabled.

When an XML parser with XML scripting support enabled creates a `script` element, it must be marked as being "parser-inserted" and its "non-blocking" flag must be unset. If the parser was originally created for the XML fragment parsing algorithm, then the element must be marked as "already started" also. When the element's end tag is subsequently parsed, the user agent must perform a microtask checkpoint, and then prepare the `script` element. If this causes there to be a pending parsing-blocking script, then the user agent must run the following steps:

1. Block this instance of the XML parser, such that the event loop will not run tasks that invoke it.

2. Spin the event loop until the parser's `Document` has no style sheet that is blocking scripts and the pending parsing-blocking script's "ready to be parser-executed" flag is set.

3. Unblock this instance of the XML parser, such that tasks that invoke it can again be run.

4. Execute the pending parsing-blocking script.

5. There is no longer a pending parsing-blocking script.

Since the `document.write()` API is not available for XML documents, much of the complexity in the HTML parser is not needed in the XML parser.

When the XML parser has XML scripting support disabled, none of this happens.

When an XML parser would append a node to a `template` element, it must instead append it to the `template` element's template contents (a `DocumentFragment` node).

This is a willful violation of the XML specification; unfortunately, XML is not formally extensible in the manner that is needed for `template` processing. [XML]

When an XML parser creates a `Node` object, its node document must be set to the node document of the node into which the newly created node is to be inserted.

Certain algorithms in this specification spoon-feed the parser characters one string at a time. In such cases, the XML parser must act as it would have if faced with a single string consisting of the concatenation of all those characters.

When an XML parser reaches the end of its input, it must stop parsing, following the same rules as the HTML parser. An XML parser can also be aborted, which must again be done in the same way as for an HTML parser.

For the purposes of conformance checkers, if a resource is determined to be in the XHTML syntax, then it is an XML document.

### 13.3 Serialising XHTML fragments

The XML fragment serialisation algorithm for a `Document` or `Element` node either returns a fragment of XML that represents that node or throws an exception.

For `Document`s, the algorithm must return a string in the form of a document entity, if none of the error cases below apply.

For `Element`s, the algorithm must return a string in the form of an internal general parsed entity, if none of the error cases below apply.

In both cases, the string returned must be XML namespace-well-formed and must be an isomorphic serialisation of all of that node's relevant child nodes, in tree order. User agents may adjust prefixes and namespace declarations in the serialisation (and indeed might be forced to do so in some cases to obtain namespace-well-formed XML). User agents may use a combination of regular text and character references to represent `Text` nodes in the DOM.

A node's relevant child nodes are those that apply given the following rules:

For `template` elements
  The relevant child nodes are the child nodes of the `template` element's template contents, if any.
For all other nodes
  The relevant child nodes are the child nodes of node itself, if any.

For `Element`s, if any of the elements in the serialisation are in no namespace, the default namespace in scope for those elements must be explicitly declared as the empty string. (This doesn't apply in the `Document` case.) [XML] [XMLNS]

For the purposes of this section, an internal general parsed entity is considered XML namespace-well-formed if a document consisting of an element with no namespace declarations whose contents are the internal general parsed entity would itself be XML namespace-well-formed.

If any of the following error cases are found in the DOM subtree being serialised, then the algorithm must throw an `InvalidStateError` exception instead of returning a string:

- A `Document` node with no child element nodes.
- A `DocumentType` node that has an external subset public identifier that contains characters that are not matched by the XML `PubidChar` production. [XML]
- A `DocumentType` node that has an external subset system identifier that contains both a U+0022 QUOTATION MARK (") and a U+0027 APOSTROPHE (') or that contains characters that are not matched by the XML `Char` production. [XML]
- A node with a local name containing a U+003A COLON (:).
- A node with a local name that does not match the XML `Name` production. [XML]
- An `Attr` node with no namespace whose local name is the lowercase string "`xmlns`". [XMLNS]
- An `Element` node with two or more attributes with the same local name and namespace.
- An `Attr` node, `Text` node, `Comment` node, or `ProcessingInstruction` node whose data contains characters that are not matched by the XML `Char` production. [XML]
- A `Comment` node whose data contains two adjacent U+002D HYPHEN-MINUS characters (-) or ends with such a character.
- A `ProcessingInstruction` node whose target name is an ASCII case-insensitive match for the string "`xml`".
- A `ProcessingInstruction` node whose target name contains a U+003A COLON (:).
- A `ProcessingInstruction` node whose data contains the string "`?>`".

These are the only ways to make a DOM unserialisable. The DOM enforces all the other XML constraints; for example, trying to append two elements to a `Document` node will throw a `HierarchyRequestError` exception.

### 13.4 Parsing XHTML fragments

The XML fragment parsing algorithm either returns a `Document` or throws a `SyntaxError` exception. Given a string input and a context element context, the algorithm is as follows:

1. Create a new XML parser.

2. Feed the parser just created the string corresponding to the start tag of the context element, declaring all the namespace prefixes that are in scope on that element in the DOM, as well as declaring the default namespace (if any) that is in scope on that element in the DOM.

   A namespace prefix is in scope if the DOM `lookupNamespaceURI()` method on the element would return a non-null value for that prefix.

   The default namespace is the namespace for which the DOM `isDefaultNamespace()` method on the element would return true.

   No `DOCTYPE` is passed to the parser, and therefore no external subset is referenced, and therefore no entities will be recognized.

3. Feed the parser just created the string input.

4. Feed the parser just created the string corresponding to the end tag of the context element.

5. If there is an XML well-formedness or XML namespace well-formedness error, then throw a `SyntaxError` exception and abort these steps.

6. If the root element of the resulting `Document` has any sibling nodes, then throw a `SyntaxError` exception and abort these steps.

7. Return the child nodes of the root element of the resulting `Document`, in tree order.