1. 2 Common infrastructure
    1. 2.1 Terminology
      1. 2.1.1 Parallelism
      2. 2.1.2 Resources
      3. 2.1.3 XML compatibility
      4. 2.1.4 DOM trees
      5. 2.1.5 Scripting
      6. 2.1.6 Plugins
      7. 2.1.7 Character encodings
      8. 2.1.8 Conformance classes
      9. 2.1.9 Dependencies
      10. 2.1.10 Extensibility
      11. 2.1.11 Interactions with XPath and XSLT
    2. 2.2 Policy-controlled features

2 Common infrastructure

This specification depends on Infra . [INFRA]

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 XML 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 XML)".

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 serialized byte streams using the HTML syntax or the XML syntax , depending on context.

In the context of the DOM structures, the terms HTML document and XML document are used as defined in DOM , 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 .

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.

2.1.1 Parallelism

To run steps in parallel means those steps are to be run, one after another, at the same time as other logic in the standard (e.g., at the same time as the event loop ). This standard 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.

For guidance on writing specifications that leverage parallelism, see Dealing with the event loop from other specifications .

To avoid race conditions between different in parallel algorithms that operate on the same data, a parallel queue can be used.

A parallel queue represents a queue of algorithm steps that must be run in series.

A parallel queue has an algorithm queue (a queue ), initially empty.

To enqueue steps to a parallel queue , enqueue the algorithm steps to the parallel queue 's algorithm queue .

To start a new parallel queue , run the following steps:

  1. Let parallelQueue be a new parallel queue .

  2. Run the following steps in parallel :

    1. While true:

      1. Let steps be the result of dequeuing from parallelQueue 's algorithm queue .

      2. If steps is not nothing, then run steps .

      3. Assert : running steps did not throw an exception, as steps running in parallel are not allowed to throw.

      Implementations are not expected to implement this as a continuously running loop. Algorithms in standards are to be easy to understand and are not necessarily great for battery life or performance.

  3. Return parallelQueue .

Steps running in parallel can themselves run other steps in in parallel . E.g., inside a parallel queue it can be useful to run a series of steps in parallel with the queue.

Imagine a standard defined nameList (a list ), along with a method to add a name to nameList , unless nameList already contains name , in which case it rejects.

The following solution suffers from race conditions:

  1. Let p be a new promise created in this 's relevant realm .

  2. Let global be this 's relevant global object .

  3. Run the following steps in parallel :

    1. If nameList contains name , then queue a global task on the DOM manipulation task source given global to reject p with a TypeError , and abort these steps.

    2. Do some potentially lengthy work.

    3. Append name to nameList .

    4. Queue a global task on the DOM manipulation task source given global to resolve p with undefined.

  4. Return p .

Two invocations of the above could run simultaneously, meaning name isn't in nameList during step 3.1, but it might be added before step 3.3 runs, meaning name ends up in nameList twice.

Parallel queues solve this. The standard would let nameListQueue be the result of starting a new parallel queue , then:

  1. Let p be a new promise created in this 's relevant realm .

  2. Let global be this 's relevant global object .

  3. Enqueue the following steps to nameListQueue :

    1. If nameList contains name , then queue a global task on the DOM manipulation task source given global to reject p with a TypeError , and abort these steps.

    2. Do some potentially lengthy work.

    3. Append name to nameList .

    4. Queue a global task on the DOM manipulation task source given global to resolve p with undefined.

  4. Return p .

The steps would now queue and the race is avoided.

2.1.2 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 specifications, refer to as a representation is referred to in this specification as a resource . [HTTP]

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.

For CSS style sheets , we tentatively define here that their critical subresources are other style sheets imported via @import rules, including those indirectly imported by other imported style sheets.

This definition is not fully interoperable; furthermore, some user agents seem to count resources like background images or web fonts as critical subresources. Ideally, the CSS Working Group would define this; see w3c/csswg-drafts issue #1088 to track progress on that front.

2.1.3 XML compatibility

To ease migration from HTML to XML, user agents 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 " refers to any element in that namespace, even in XML documents.

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 .

2.1.4 DOM trees

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 for an attribute value, Text node, or string, means that the length of the text is zero (i.e., not even containing controls or U+0020 SPACE).

An HTML element can have specific HTML element insertion steps , HTML element post-connection steps , HTML element removing steps , and HTML element moving steps all defined for the element's local name .

The insertion steps for the HTML Standard, given insertedNode , are defined as the following:

  1. If insertedNode is an element whose namespace is the HTML namespace , and this standard defines HTML element insertion steps for insertedNode 's local name , then run the corresponding HTML element insertion steps given insertedNode .

  2. If insertedNode is a form-associated element or the ancestor of a form-associated element , then:

    1. If the form-associated element 's parser inserted flag is set, then return.

    2. Reset the form owner of the form-associated element .

  3. If insertedNode is an Element that is not on the stack of open elements of an HTML parser , then process internal resource links given insertedNode 's node document .

The post-connection steps for the HTML Standard, given insertedNode , are defined as the following:

  1. If insertedNode is an element whose namespace is the HTML namespace , and this standard defines HTML element post-connection steps for insertedNode 's local name , then run the corresponding HTML element post-connection steps given insertedNode .

The removing steps for the HTML Standard, given removedNode and oldParent , are defined as the following:

  1. Let document be removedNode 's node document .

  2. If document 's focused area is removedNode , then set document 's focused area to document 's viewport , and set document 's relevant global object 's navigation API 's focus changed during ongoing navigation to false.

    This does not perform the unfocusing steps , focusing steps , or focus update steps , and thus no blur or change events are fired.

  3. If removedNode is an element whose namespace is the HTML namespace , and this standard defines HTML element removing steps for removedNode 's local name , then run the corresponding HTML element removing steps given removedNode and oldParent .

  4. If removedNode is a form-associated element with a non-null form owner and removedNode and its form owner are no longer in the same tree , then reset the form owner of removedNode .

  5. If removedNode 's popover attribute is not in the No Popover state, then run the hide popover algorithm given removedNode , false, false, false, and null.

  6. If removedNode is an HTML element with a non-null active interest target , then reset interest state for removedNode .

  7. If removedNode is an element with a non-null active interest source , then reset interest state for removedNode 's active interest source .

The moving steps for the HTML Standard, given movedNode , are defined as the following:

  1. If movedNode is an element whose namespace is the HTML namespace , and this standard defines HTML element moving steps for movedNode 's local name , then run the corresponding HTML element moving steps given movedNode .

  2. If movedNode is a form-associated element with a non-null form owner and movedNode and its form owner are no longer in the same tree , then reset the form owner of movedNode .

  3. If movedNode is an HTML element with a non-null active interest target and movedNode and its active interest target are no longer in the same tree , then reset interest state for movedNode .

  4. If movedNode is an element with a non-null active interest source and movedNode and its active interest source are no longer in the same tree , then reset interest state for movedNode 's active interest source .

A node is inserted into a document when the insertion steps are invoked with it as the argument and it is now in a document tree . Analogously, a node is removed from a document when the removing steps are invoked with it as the argument and it is now no longer in a document tree .

A node becomes connected when the insertion steps are invoked with it as the argument and it is now connected . Analogously, a node becomes disconnected when the removing steps are invoked with it as the argument and it is now no longer connected .

A node is browsing-context connected when it is connected and its shadow-including root 's browsing context is non-null. A node becomes browsing-context connected when the insertion steps are invoked with it as the argument and it is now browsing-context connected . A node becomes browsing-context disconnected either when the removing steps are invoked with it as the argument and it is now no longer browsing-context connected , or when its shadow-including root 's browsing context becomes null.

2.1.5 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.

2.1.6 Plugins

The term plugin refers to an implementation-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 navigables 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 navigable 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]

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.

(This is a tracking vector.) Since different users having different sets of plugins provides a tracking 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.

2.1.7 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 Encoding . 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]

2.1.8 Conformance classes

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

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 XML 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 web browser would, upon finding a 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 Web IDL . [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 XML 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 color 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 the Rendering section defines as the behavior 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 XML 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 headings 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 its nearest article or body element ancestor. 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 an article element, 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.

For compatibility with existing content and prior specifications, this specification describes two authoring formats: one based on XML , 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 behavior. 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.)

2.1.9 Dependencies

This specification relies on several other underlying specifications.

Infra

The following terms are defined in Infra : [INFRA]

Unicode and Encoding

The Unicode character set is used to represent textual data, and Encoding 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 Encoding : [ENCODING]

  • Getting an encoding
  • Get an output encoding
  • 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 decode without BOM algorithm which is identical to UTF-8 decode except that it does not strip one leading UTF-8 Byte Order Mark (BOM)
  • The encode algorithm which takes a character stream and an encoding and returns a byte stream
  • The UTF-8 encode algorithm which takes a character stream and returns a byte stream
  • The BOM sniff algorithm which takes a byte stream and returns an encoding or null.
XML and related specifications

Implementations that support the XML syntax for HTML must support some version of XML, as well as its corresponding namespaces specification, because that syntax uses an XML serialization with namespaces. [XML] [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 namespace 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.

The attribute with the name space in the XML namespace is defined by Extensible Markup Language ( XML ). [XML]

The Name production is defined in XML . [XML]

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

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

URLs

The following terms are defined in URL : [URL]

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

Media fragment syntax is defined in Media Fragments URI . [MEDIAFRAG]

URL Pattern

The following terms are defined in URL Pattern : [URLPATTERN]

HTTP and related specifications

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

The following terms are defined in HTTP State Management Mechanism : [COOKIES]

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

The following terms are defined in Structured Field Values for HTTP : [STRUCTURED-FIELDS]

The following terms are defined in MIME Sniffing : [MIMESNIFF]

Fetch

The following terms are defined in Fetch : [FETCH]

The following terms are defined in Referrer Policy : [REFERRERPOLICY]

The following terms are defined in Mixed Content : [MIX]

The following terms are defined in Subresource Integrity : [SRI]

The No-Vary-Search HTTP Response Header Field

The following terms are defined in The No-Vary-Search HTTP Response Header Field : [NOVARYSEARCH]

Paint Timing

The following terms are defined in Paint Timing : [PAINTTIMING]

Navigation Timing

The following terms are defined in Navigation Timing : [NAVIGATIONTIMING]

Resource Timing

The following terms are defined in Resource Timing : [RESOURCETIMING]

Performance Timeline

The following terms are defined in Performance Timeline : [PERFORMANCETIMELINE]

Long Animation Frames

The following terms are defined in Long Animation Frames : [LONGANIMATIONFRAMES]

Long Tasks

The following terms are defined in Long Tasks : [LONGTASKS]

Web IDL

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

The following terms are defined in Web IDL :

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

The term throw in this specification is used as defined in Web IDL . The DOMException type and the following exception names are defined by Web IDL and used by this specification:

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. [JAVASCRIPT]

The term "JavaScript" is used to refer to ECMA-262, rather than the official term ECMAScript, since the term JavaScript is more widely known.

The following terms are defined in the JavaScript specification and used in this specification: