HTML

1. 2 Common infrastructure
   1. 2.1 Terminology
      1. 2.1.1 Resources
      2. 2.1.2 XML compatibility
      3. 2.1.3 DOM trees
      4. 2.1.4 Scripting
      5. 2.1.5 Plugins
      6. 2.1.6 Character encodings
      7. 2.1.7 Conformance classes
      8. 2.1.8 Dependencies
      9. 2.1.9 Extensibility
      10. 2.1.10 Interactions with XPath and XSLT
   2. 2.2 Case-sensitivity and string comparison
   3. 2.3 Common microsyntaxes
      1. 2.3.1 Common parser idioms
      2. 2.3.2 Boolean attributes
      3. 2.3.3 Keywords and enumerated attributes
      4. 2.3.4 Numbers
         1. 2.3.4.1 Signed integers
         2. 2.3.4.2 Non-negative integers
         3. 2.3.4.3 Floating-point numbers
         4. 2.3.4.4 Percentages and lengths
         5. 2.3.4.5 Non-zero percentages and lengths
         6. 2.3.4.6 Lists of floating-point numbers
         7. 2.3.4.7 Lists of dimensions
      5. 2.3.5 Dates and times
         1. 2.3.5.1 Months
         2. 2.3.5.2 Dates
         3. 2.3.5.3 Yearless dates
         4. 2.3.5.4 Times
         5. 2.3.5.5 Local dates and times
         6. 2.3.5.6 Time zones
         7. 2.3.5.7 Global dates and times
         8. 2.3.5.8 Weeks
         9. 2.3.5.9 Durations
         10. 2.3.5.10 Vaguer moments in time
      6. 2.3.6 Colors
      7. 2.3.7 Space-separated tokens
      8. 2.3.8 Comma-separated tokens
      9. 2.3.9 References
      10. 2.3.10 Media queries
   4. 2.4 URLs
      1. 2.4.1 Terminology
      2. 2.4.2 Parsing URLs
      3. 2.4.3 Dynamic changes to base URLs
   5. 2.5 Fetching resources
      1. 2.5.1 Terminology
      2. 2.5.2 Determining the type of a resource
      3. 2.5.3 Extracting character encodings from meta elements
      4. 2.5.4 CORS settings attributes
      5. 2.5.5 Referrer policy attributes
   6. 2.6 Common DOM interfaces
      1. 2.6.1 Reflecting content attributes in IDL attributes
      2. 2.6.2 Collections
         1. 2.6.2.1 The HTMLAllCollection interface
         2. 2.6.2.2 The HTMLFormControlsCollection interface
         3. 2.6.2.3 The HTMLOptionsCollection interface
      3. 2.6.3 The DOMStringList interface
      4. 2.6.4 Garbage collection
   7. 2.7 Safe passing of structured data
      1. 2.7.1 Cloneable objects
      2. 2.7.2 Transferable objects
      3. 2.7.3 StructuredCloneWithTransfer ( input, transferList, targetRealm )
      4. 2.7.4 StructuredClone ( input, targetRealm [ , memory ] )
      5. 2.7.5 IsTransferable ( O )
      6. 2.7.6 Transfer ( input, targetRealm )
      7. 2.7.7 Performing structured clones from other specifications
      8. 2.7.8 Monkey patch for Blob and FileList objects

2 Common infrastructure

This specification depends on the WHATWG Infra standard. [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 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.

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 color 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]

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 compatibility

To ease migration from HTML to XML, 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, 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.

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]

2.1.3 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 spaces or control characters).

An element's child text content is the concatenation of the data of all the Text nodes that are children of the element (ignoring any other nodes such as comments or elements), in tree order.

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.

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 has a browsing context. 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 no longer has a browsing context.

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.

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 different 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]

A UTF-16 encoding is UTF-16BE or UTF-16LE. [ENCODING]

An ASCII-compatible encoding is any encoding that is not a UTF-16 encoding. [ENCODING]

Since support for encodings that are not defined in the WHATWG Encoding standard is prohibited, UTF-16 encodings are the only encodings that this specification needs to treat as not being ASCII-compatible encodings.

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.

2.1.7 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 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 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 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 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, 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.)

2.1.8 Dependencies

2.1.9 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.

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

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 recognize 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.

2.1.10 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 serialize 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.

2.2 Case-sensitivity and string comparison

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

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

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.

2.3 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 behavior that differs from what is required in this specification, since error-handling behavior 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.

2.3.1 Common parser idioms

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

This is not to 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]

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.

2.3.2 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.

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

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

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

2.3.3 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.

2.3.4 Numbers

2.3.4.1 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 ASCII whitespace within input given position.

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 code points that are ASCII digits from input given position, 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.

2.3.4.2 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.

2.3.4.3 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. Both of the following, in the given order:
      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 valid floating-point number concept is typically only used to restrict what is allowed for authors, while the user agent requirements use the rules for parsing floating-point number values below (e.g., the max attribute of the progress element). However, in some cases the user agent requirements include checking if a string is a valid floating-point number (e.g., the value sanitization algorithm for the Number state of the input element, or the parse a srcset attribute algorithm).

The best representation of the number n as a floating-point number is the string obtained from running ToString(n). The abstract operation ToString is not uniquely determined. When there are multiple possible strings that could be obtained from 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 ASCII whitespace within input given position.

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 code points that are ASCII digits from input given position, 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 code points that are ASCII digits from input given position, 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.

2.3.4.4 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 0.0, or an error; if a number is returned, then it is further categorized 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 ASCII whitespace within input given position.

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. 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 code points that are ASCII digits from input given position, and interpret the resulting sequence as a base-ten integer. Let value be that number.

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

10. 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.

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

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

13. Return value as a length.

2.3.4.5 Non-zero percentages and lengths

The rules for parsing non-zero 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 0.0, or an error; if a number is returned, then it is further categorized as either a percentage or a length.

1. Let input be the string being parsed.

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

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

4. If value is zero, return an error.

5. If value is a percentage, return value as a percentage.

6. Return value as a length.

2.3.4.6 Lists of floating-point numbers

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

The rules for parsing a list of floating-point numbers 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 floating-point numbers. This list will be the result of this algorithm.

4. Collect a sequence of code points that are ASCII whitespace, U+002C COMMA, or U+003B SEMICOLON characters from input given position. This skips past any leading delimiters.

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

   1. Collect a sequence of code points that are not ASCII whitespace, U+002C COMMA, U+003B SEMICOLON, ASCII digits, U+002E FULL STOP, or U+002D HYPHEN-MINUS characters from input given position. This skips past leading garbage.

   2. Collect a sequence of code points that are not ASCII whitespace, U+002C COMMA, or U+003B SEMICOLON characters from input given position, and let unparsed number be the result.

   3. Let number be the result of parsing unparsed number using the rules for parsing floating-point number values.

   4. If number is an error, set number to zero.

   5. Append number to numbers.

   6. Collect a sequence of code points that are ASCII whitespace, U+002C COMMA, or U+003B SEMICOLON characters from input given position. This skips past the delimiter.

6. Return numbers.

2.3.4.7 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 code points that are ASCII digits from input given position, 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 code points consisting of ASCII whitespace and ASCII digits from input given position. Let s be the resulting sequence.

      2. Remove all ASCII whitespace 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 10^length.

         3. Increment value by fraction.

   8. Skip ASCII whitespace within input given position.

   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.

2.3.5 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), implementation notes regarding localization of form controls, and the time element.

2.3.5.1 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 code points that are ASCII digits from input given position. 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 code points that are ASCII digits from input given position. 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.

2.3.5.2 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 code points that are ASCII digits from input given position. 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.

2.3.5.3 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 code points that are U+002D HYPHEN-MINUS characters (-) from input given position. If the collected sequence is not exactly zero or two characters long, then fail.

2. Collect a sequence of code points that are ASCII digits from input given position. 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 code points that are ASCII digits from input given position. 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.

2.3.5.4 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 U+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 code points that are ASCII digits from input given position. 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 code points that are ASCII digits from input given position. 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 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 code points that are either ASCII digits or U+002E FULL STOP characters from input given position. 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, interpret the resulting sequence as a base-ten number (possibly with a fractional part). Set second to that number.

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

8. Return hour, minute, and second.

2.3.5.5 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 normalized 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.

2.3.5.6 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 timezone_hours and timezone_minutes. 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 timezone_hours hours and timezone_minutes 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 timezone_hours be 0.

   2. Let timezone_minutes 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 code points that are ASCII digits from input given position. 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 timezone_hours.

      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 code points that are ASCII digits from input given position. 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 timezone_minutes.

      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 timezone_hours.

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

      Otherwise, fail.

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

   Otherwise, fail.

2. Return timezone_hours and timezone_minutes.

2.3.5.7 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 interpreted 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 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 timezone_hours and timezone_minutes. 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 timezone_hours hours and timezone_minutes minutes. That moment in time is a moment in the UTC time zone.

10. Let timezone be timezone_hours hours and timezone_minutes minutes from UTC.

11. Return time and timezone.

2.3.5.8 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 code points that are ASCII digits from input given position. 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 code points that are ASCII digits from input given position. 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.

2.3.5.9 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 ASCII whitespace.

  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 ASCII whitespace.

  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 ASCII whitespace.

  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 ASCII whitespace within input given position.

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 ASCII whitespace within input given position.

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 ASCII whitespace within input given position, 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 code points that are ASCII digits from input given position, 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 code points that are ASCII digits from input given position. 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 10^length.

      5. Increment N by fraction.

      6. Skip ASCII whitespace within input given position.

      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 ASCII whitespace, then skip ASCII whitespace within input given position, 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 ASCII whitespace within input given position.

9. If component count is zero, fail.

10. If months is not zero, fail.

11. Return the duration consisting of seconds seconds.

2.3.5.10 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 timezone_hours and timezone_minutes. 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 timezone_hours hours and timezone_minutes minutes, that moment in time being a moment in the UTC time zone; let timezone be timezone_hours hours and timezone_minutes minutes from UTC; and return time and timezone.

2.3.6 Colors

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

A string is a valid simple color 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 color if it is a valid simple color 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 color 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 color 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 color.

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 serializing simple color values given a simple color 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 ASCII lower 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 color.

Some obsolete legacy attributes parse colors in a more complicated manner, using the rules for parsing a legacy color 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 color 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 ASCII 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 colors, then return the simple color corresponding to that keyword. [CSSCOLOR]

   CSS2 System Colors are not recognized.

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 color.

   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 color.

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 color syntax that also handles opacity.

2.3.7 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 ASCII whitespace, where words consist of any string of one or more characters, none of which are ASCII whitespace.

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

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.

2.3.8 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 ASCII whitespace, nor containing any U+002C COMMA characters (,), and optionally surrounded by ASCII whitespace.

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.

2.3.9 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 same tree.

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 scope's tree, in tree order, that has an id or name attribute whose value is s, or null if there is no such element.

   Although id attributes are accounted for when parsing, they are not used in determining whether a value is a valid hash-name reference. That is, a hash-name reference that refers to an element based on id is a conformance error (unless that element also has a name attribute with the same value).

2.3.10 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 ASCII whitespace, or is a media query list that matches the user's environment according to the definitions given in the Media Queries specification. [MQ]

2.4 URLs

2.4.1 Terminology

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

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

A string is a valid non-empty URL potentially surrounded by spaces if, after stripping leading and trailing ASCII 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:html-kind as a reserved, though unresolvable, about: URL, that is used as an identifier for kinds of media tracks. [ABOUT]

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

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

1. If document is an iframe srcdoc document, then return the document base URL of document's browsing context's browsing context container's node document.

2. If document's URL is about:blank, and document's browsing context has a creator browsing context, then return the creator base URL.

3. Return document's URL.

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.

2.4.2 Parsing URLs

Parsing a URL is the process of taking a string and obtaining the URL record that it represents. While this process is defined in the WHATWG URL standard, the HTML standard defines a wrapper for convenience. [URL]

This wrapper is only useful when the character encoding for the URL parser has to match that of the document or environment settings object for legacy reasons. When that is not the case the URL parser can be used directly.

To parse a URL url, relative to either a document or environment settings object, the user agent must use the following steps. Parsing a URL either results in failure or a resulting URL string and resulting URL record.

1. Let encoding be document's character encoding, if document was given, and environment settings object's API URL character encoding otherwise.

2. Let baseURL be document's base URL, if document was given, and environment settings object's API base URL otherwise.

3. Let urlRecord be the result of applying the URL parser to url, with baseURL and encoding.

4. If urlRecord is failure, then abort these steps with an error.

5. Let urlString be the result of applying the URL serializer to urlRecord.

6. Return urlString as the resulting URL string and urlRecord as the resulting URL record.

2.4.3 Dynamic changes to base URLs

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 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 reparsed relative to the element's node document 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 URL(s) are being shown to the user, then the ping attribute's tokens should be reparsed relative to the element's node document and the UI updated appropriately.

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

If the 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 reparsed relative to the element's node document 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.

2.5 Fetching resources

2.5.1 Terminology

A response whose type is "basic", "cors", or "default" is CORS-same-origin. [FETCH]

A response whose type is "opaque" or "opaqueredirect" is CORS-cross-origin.

A response's unsafe response is its internal response if it has one, and the response itself otherwise.

To create a potential-CORS request, given a url, destination, corsAttributeState, and an optional same-origin fallback flag, run these steps:

1. Let mode be "no-cors" if corsAttributeState is No CORS, and "cors" otherwise.

2. If same-origin fallback flag is set and mode is "no-cors", set mode to "same-origin".

3. Let credentialsMode be "include".

4. If corsAttributeState is Anonymous, set credentialsMode to "same-origin".

5. Let request be a new request whose url is url, destination is destination, mode is mode, credentials mode is credentialsMode, and whose use-URL-credentials flag is set.

2.5.2 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 WHATWG MIME Sniffing standard. [MIMESNIFF]

The computed MIME type of a resource must be found in a manner consistent with the requirements given in the WHATWG MIME Sniffing standard. [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 WHATWG MIME Sniffing standard. These rules return a MIME type as their result. [MIMESNIFF]

It is imperative that the rules in the WHATWG MIME Sniffing standard 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 WHATWG MIME Sniffing standard. [MIMESNIFF]

2.5.3 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 ASCII whitespace 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 ASCII whitespace 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 ASCII whitespace 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]

2.5.4 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.

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.

2.5.5 Referrer policy attributes

A referrer policy attribute is an enumerated attribute. Each referrer policy, including the empty string, is a keyword for this attribute, mapping to a state of the same name.

The attribute's invalid value default and missing value default are both the empty string state.

The impact of these states on the processing model of various fetches is defined in more detail throughout this specification, in the WHATWG Fetch standard, and in Referrer Policy. [FETCH] [REFERRERPOLICY]

2.6 Common DOM interfaces

2.6.1 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 USVString attribute whose content attribute is defined to contain a URL, then on getting, if the content attribute is absent, the IDL attribute must return the empty string. Otherwise, the IDL attribute must parse the value of the content attribute relative to the element's node document and if that is successful, return the resulting URL string. If parsing fails, then the value of the content attribute must be returned instead, converted to a USVString. On setting, the content attribute must be set to the specified new 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). 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). 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 or USVString 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 an IDL enumeration attribute, then, on getting, if the corresponding content attribute's value case-sensitively matches one of the enumerated values, then the IDL attribute must return the content attribute's value; otherwise it must return the content attribute's default value. On setting, the content attribute must be set to the specified new value.

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" DOMException. 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 behavior 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" DOMException. 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 an unsigned integer type (unsigned long) that is limited to only non-negative numbers greater than zero with fallback, then the behavior is similar to the previous case, but disallowed values are converted to the default value. 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. On setting, 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; 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 behavior 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, then on getting it must return a DOMTokenList object whose associated element is the element in question and whose associated attribute's local name is the name of the attribute in question.

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 tree as the element of the attribute being set, and the given element is the first element in that tree 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.

2.6.2 Collections

The HTMLFormControlsCollection and HTMLOptionsCollection interfaces are collections derived from the HTMLCollection interface. The HTMLAllCollection interface is a collection, but is not so derived.

2.6.2.1 The HTMLAllCollection interface

The HTMLAllCollection interface is used for the legacy document.all attribute. It operates similarly to HTMLCollection; the main differences are that it allows a staggering variety of different (ab)uses of its methods to all end up returning something, and that it can be called as a function as an alternative to property access.

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.

[LegacyUnenumerableNamedProperties]
interface HTMLAllCollection {
  readonly attribute unsigned long length;
  getter Element? (unsigned long index);
  getter (HTMLCollection or Element)? namedItem(DOMString name);
  legacycaller (HTMLCollection or Element)? item(optional DOMString nameOrIndex);
};

collection . length

Returns the number of elements in the collection.

element = collection . item(index)

element = collection(index)

element = collection[index]

Returns the item with index index from the collection (determined by tree order).

element = collection . item(name)

collection = collection . item(name)

element = collection . namedItem(name)

collection = collection . namedItem(name)

element = collection(name)

collection = collection(name)

element = collection[name]

collection = 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 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.

On getting, the length attribute must return the number of nodes represented by the collection.

The indexed property getter must return the result of getting the "all"-indexed element from this HTMLAllCollection given the passed index.

The namedItem(name) method must return the result of getting the "all"-named element(s) from this HTMLAllCollection given name.

The item(nameOrIndex) method (and the legacycaller behavior) must run the following steps:

1. If nameOrIndex was not provided, return null.

2. If nameOrIndex, converted to a JavaScript string value, is an array index property name, return the result of getting the "all"-indexed element from this HTMLAllCollection given the number represented by nameOrIndex.

3. Return the result of getting the "all"-named element(s) from this HTMLAllCollection given nameOrIndex.

The following elements are "all"-named elements: a, applet, button, embed, form, frame, frameset, iframe, img, input, map, meta, object, select, and textarea

To get the "all"-indexed element from an HTMLAllCollection collection given an index index, return the index^th element in collection, or null if there is no such index^th element.

To get the "all"-named element(s) from an HTMLAllCollection collection given a name name, perform the following steps:

1. If name is the empty string, return null.

2. Let subCollection be an HTMLCollection object rooted at the same Document as collection, 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 there is exactly one element in subCollection, then return that element.

4. Otherwise, if subCollection is empty, return null.

5. Otherwise, return subCollection.

2.6.2.2 The HTMLFormControlsCollection interface

The HTMLFormControlsCollection interface is used for collections of listed elements in form elements.

interface HTMLFormControlsCollection : HTMLCollection {
  // inherits length and item()
  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)

element = 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)

element = collection[name]

radioNodeList = 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 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.

2.6.2.3 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(), namedItem()
  [CEReactions] attribute unsigned long length; // shadows inherited length
  [CEReactions] setter void (unsigned long index, HTMLOptionElement? option);
  [CEReactions] void add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
  [CEReactions] 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)

element = collection[index]

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

collection[index] = element

When index is a greater number than the number of items in the collection, adds new blank option elements in the corresponding container.

When set to null, removes the item at index index from the collection.

When set to an option element, adds or replaces it at index index from the collection.

element = collection . namedItem(name)

element = 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" DOMException if element is an ancestor of the element into which it is to be inserted.

collection . remove(index)

Removes the item with index index from the collection.

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 behavior 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.

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" DOMException 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" DOMException 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. Pre-insert element into parent node before reference.

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

2.6.3 The DOMStringList interface

The DOMStringList interface is a non-fashionable retro way of representing a list of strings.

[Exposed=(Window,Worker)]
interface DOMStringList {
  readonly attribute unsigned long length;
  getter DOMString? item(unsigned long index);
  boolean contains(DOMString string);
};

New APIs must use sequence<DOMString> or equivalent rather than DOMStringList.

strings . length

Returns the number of strings in strings.

strings[index]

strings . item(index)

Returns the string with index index from strings.

strings . contains(string)

Returns true if strings contains string, and false otherwise.

Each DOMStringList object has an associated list.

The supported property indices for a DOMStringList object are the numbers zero to the associated list's size minus one. If its associated list is empty, it has no supported property indices.

The length attribute's getter must this DOMStringList object's associated list's size.

The item(index) method, when invoked, must return the indexth item in this DOMStringList object's associated list, or null if index plus one is less than this DOMStringList object's associated list's size.

The contains(string) method, when invoked, must return true if this DOMStringList object's associated list contains string, and false otherwise.

2.6.4 Garbage collection

There is an implied strong reference from any IDL attribute that returns a pre-existing object to that object.

2.7 Safe passing of structured data

This section uses the terminology and typographic conventions from the JavaScript specification. [JAVASCRIPT]

2.7.1 Cloneable objects

Cloneable objects support being cloned across event loops. That is, they support being cloned across document and worker boundaries, including across documents of different origins. Not all objects are cloneable objects and not all aspects of objects that are cloneable objects are necessarily preserved when cloned.

Platform objects have the following internal method:

[[Clone]] ( targetRealm, memory )

Unless specified otherwise, invoking the [[Clone]]() internal method must throw a "DataCloneError" DOMException. (By default, platform objects are not cloneable objects.)

Platform objects that are cloneable objects have a [[Clone]]() internal method which is specified to run a series of steps. The result of running those steps must be a thrown exception or a clone of this, created in targetRealm. It is up such objects to define what cloning means for them.

Objects defined in the JavaScript specification are handled by the StructuredClone abstract operation directly.

2.7.2 Transferable objects

Transferable objects support being transferred across event loops. Transferring is effectively recreating the object while sharing a reference to the underlying data and then detaching the object being transferred. This is useful to transfer ownership of expensive resources. Not all objects are transferable objects and not all aspects of objects that are transferable objects are necessarily preserved when transferred.

Transferring is an irreversible and non-idempotent operation. Once an object has been transferred, it cannot be transferred, or indeed used, again.

Platform objects that are transferable objects have a [[Detached]] internal slot and the following internal method:

[[Transfer]]( targetRealm )

Whereas all platform objects have a [[Clone]]() internal method, not all have a [[Detached]] internal slot and a [[Transfer]]() internal method.

Platform objects that are transferable objects must define the [[Transfer]]() internal method such that it either throws an exception or returns a clone of this, created in targetRealm, with this's underlying data shared with the return value, and this's [[Detached]] internal slot value set to true. It is up to such objects to define what transferring means for them.

Objects defined in the JavaScript specification are handled by the StructuredCloneWithTransfer abstract operation directly. (Technically, by IsTransferable and Transfer.)

2.7.3 StructuredCloneWithTransfer ( input, transferList, targetRealm )

1. Let memory be an empty map.

   The purpose of the memory map, both here and in the StructuredClone abstract operation, is to avoid cloning objects twice. This ends up preserving cycles and the identity of duplicate objects in graphs.

2. For each object transferable in transferList:

   1. If IsTransferable(transferable) is false, then throw a "DataCloneError" DOMException.

   2. Let placeholder be a user-agent-defined placeholder object.

   3. Create an entry in memory with key transferable and value placeholder.

3. Let clone be the result of ? StructuredClone(input, targetRealm, memory).

4. Let outputTransferList be a new empty List.

5. For each object transferable in transferList:

   1. Let placeholderResult be the value of the entry in memory whose key is transferable.

   2. Let transferResult be ? Transfer(transferable, targetRealm).

   3. Within clone, replace references to placeholderResult with transferResult, such that everything holding a reference to placeholderResult, now holds a reference to transferResult.

      This is a rather unusual low-level operation for which no primitives are defined by JavaScript.

   4. Add transferResult as the last element of outputTransferList.

6. Return { [[Clone]]: clone, [[TransferList]]: outputTransferList }.

Originally the StructuredCloneWithTransfer abstract operation was known as the "structured clone" algorithm. The StructuredClone abstract operation was known as the "internal structured clone" algorithm. Transferring objects, now handled by the StructuredCloneWithTransfer abstract operation, were formerly handled by parts of the algorithm of the postMessage() method on the Window object and the postMessage() method on the MessagePort object.

2.7.4 StructuredClone ( input, targetRealm [ , memory ] )

1. If memory was not supplied, let memory be an empty map.

2. If memory contains an entry with key input, then return that entry's value.

3. If Type(input) is Undefined, Null, Boolean, String, or Number, then return input.

4. If Type(input) is Symbol, then throw a "DataCloneError" DOMException.

5. Let deepClone be false.

6. If input has a [[BooleanData]] internal slot, then let output be a new Boolean object in targetRealm whose [[BooleanData]] internal slot value is the [[BooleanData]] internal slot value of input.

7. Otherwise, if input has a [[NumberData]] internal slot, then let output be a new Number object in targetRealm whose [[NumberData]] internal slot value is the [[NumberData]] internal slot value of input.

8. Otherwise, if input has a [[StringData]] internal slot, then let output be a new String object in targetRealm whose [[StringData]] internal slot value is the [[StringData]] internal slot value of input.

9. Otherwise, if input has a [[DateValue]] internal slot, then let output be a new Date object in targetRealm whose [[DateValue]] internal slot value is the [[DateValue]] internal slot value of input.

10. Otherwise, if input has a [[RegExpMatcher]] internal slot, then let output be a new RegExp object in targetRealm whose [[RegExpMatcher]] internal slot value is the [[RegExpMatcher]] internal slot value of input, whose [[OriginalSource]] internal slot value is the [[OriginalSource]] internal slot value of input, and whose whose [[OriginalFlags]] internal slot value is the [[OriginalFlags]] internal slot value of input.

11. Otherwise, if input has an [[ArrayBufferData]] internal slot, then:

    1. If IsDetachedBuffer(input) is true, then throw a "DataCloneError" DOMException.

    2. Let outputArrayBuffer be the %ArrayBuffer% intrinsic object in targetRealm.

    3. Let output be ? CloneArrayBuffer(input, 0, outputArrayBuffer).

12. Otherwise, if input has a [[ViewedArrayBuffer]] internal slot, then:

    1. Let buffer be the value of input's [[ViewedArrayBuffer]] internal slot.

    2. Let bufferClone be ? StructuredClone(buffer, targetRealm, memory).

    3. If input has a [[DataView]] internal slot, then let output be a new DataView object in targetRealm whose [[DataView]] internal slot value is true, whose [[ViewedArrayBuffer]] internal slot value is bufferClone, whose [[ByteLength]] internal slot value is the [[ByteLength]] internal slot value of input, and whose [[ByteOffset]] internal slot value is the [[ByteOffset]] internal slot value of input.

    4. Otherwise:

       1. Assert: input has a [[TypedArrayName]] internal slot.

       2. Let constructor be the intrinsic object listed in column one of The TypedArray Constructors table for the value of input's [[TypedArrayName]] internal slot in targetRealm.

       3. Let byteOffset be input's [[ByteOffset]] internal slot value.

       4. Let length be input's [[ArrayLength]] internal slot value.

       5. Let output be ? TypedArrayCreate(constructor, « bufferClone, byteOffset, length »).

13. Otherwise, if input has [[MapData]] internal slot, then:

    1. Let output be a new Map object in targetRealm whose [[MapData]] internal slot value is a new empty List.

    2. Set deepClone to true.

14. Otherwise, if input has [[SetData]] internal slot, then:

    1. Let output be a new Set object in targetRealm whose [[SetData]] internal slot value is a new empty List.

    2. Set deepClone to true.

15. Otherwise, if input is an Array exotic object, then:

    1. Let inputLen be OrdinaryGetOwnProperty(input, "length").[[Value]].

    2. Let outputProto be the %ArrayPrototype% intrinsic object in targetRealm.

    3. Let output be ! ArrayCreate(inputLen, outputProto).

    4. Set deepClone to true.

16. Otherwise, if input has a [[Clone]]() internal method, then let output be ? input.[[Clone]](targetRealm, memory).

17. Otherwise, if IsCallable(input) is true, then throw a "DataCloneError" DOMException.

18. Otherwise, if input has any internal slot other than [[Prototype]] or [[Extensible]], then throw a "DataCloneError" DOMException.

    For instance, a [[PromiseState]] or [[WeakMapData]] internal slot.

19. Otherwise, if input is an exotic object, then throw a "DataCloneError" DOMException.

    For instance, a proxy object.

20. Otherwise:

    1. Let output be a new Object in targetRealm.

    2. Set deepClone to true.

21. Create an entry in memory whose key is input and value is output.

22. If deepClone is true, then:

    1. If input has a [[MapData]] internal slot, then:

       1. Let inputList the value of input's [[MapData]] internal slot.

       2. Let copiedList be a new empty List.

       3. Repeat for each Record { [[Key]], [[Value]] } entry that is an element of inputList,

          1. Let copiedEntry be a new Record { [[Key]]: entry.[[Key]], [[Value]]: entry.[[Value]] }.

          2. If copiedEntry.[[Key]] is not empty, append copiedEntry as the last element of copiedList.

       4. Let outputList be the value of output's [[MapData]] internal slot.

       5. For each Record { [[Key]], [[Value]] } entry that is an element of copiedList,

          1. Let outputKey be ? StructuredClone(entry.[[Key]], targetRealm, memory).

          2. Let outputValue be ? StructuredClone(entry.[[Value]], targetRealm, memory).

          3. Add { [[Key]]: outputKey, [[Value]]: outputValue } as the last element of outputList.

    2. Otherwise, if input has a [[SetData]] internal slot, then:

       1. Let copiedList be a copy of the value of input's [[SetData]] internal slot.

       2. Let outputList be the value of output's [[SetData]] internal slot.

       3. For each entry that is an element of copiedList that is not empty,

          1. Let outputEntry be ? StructuredClone(entry, targetResult, memory).

          2. Add outputEntry as the last element of outputList.

    3. Otherwise:

       1. Let enumerableKeys be a new empty List.

       2. For each key in ! input.[[OwnPropertyKeys]]():

          1. If Type(key) is String, then:

             1. Let inputDesc be ! input.[[GetOwnProperty]](key).

             2. If inputDesc.[[Enumerable]] is true, then add key as the last element of enumerableKeys.

       3. For each key in enumerableKeys:

          1. If ! HasOwnProperty(input, key) is true, then:

             1. Let inputValue be ? input.[[Get]](key, input).

             2. Let outputValue be ? StructuredClone(inputValue, targetRealm, memory).

             3. Perform ? CreateDataProperty(output, key, outputValue).

       The key collection performed above is very similar to the JavaScript specification's EnumerableOwnProperties operation, but crucially it uses the deterministic ordering provided by the [[OwnPropertyKeys]] internal method, instead of reordering the keys in an unspecified manner as EnumerableOwnProperties does. [JAVASCRIPT]

23. Return output.

In general implementations will need to use some kind of serialization and marshalling to implement the creation of objects in targetRealm, as targetRealm could be in a different event loop and not easily accessible to the code that invokes StructuredCloneWithTransfer or StructuredClone.

2.7.5 IsTransferable ( O )

1. Assert: Type(O) is Object.

2. If O has an [[ArrayBufferData]] internal slot, then:

   1. If IsDetachedBuffer(O) is true, then return false.

   2. Return true.

3. Otherwise, if O has a [[Detached]] internal slot, then:

   1. If O's [[Detached]] internal slot value is true, then return false.

   2. Return true.

4. Return false.

2.7.6 Transfer ( input, targetRealm )

1. If input has an [[ArrayBufferData]] internal slot, then:

   1. Let output be a new ArrayBuffer object in targetRealm whose [[ArrayBufferByteLength]] internal slot value is the [[ArrayBufferByteLength]] internal slot value of input, and whose [[ArrayBufferData]] internal slot value is the [[ArrayBufferData]] internal slot value of input.

   2. Perform ! DetachArrayBuffer(input).

   3. Return output.

2. Return ? input.[[Transfer]](targetRealm).

2.7.7 Performing structured clones from other specifications

Other specifications may use the StructuredClone, StructuredCloneWithTransfer, IsTransferable, and Transfer abstract operations.

In general, call sites may pass in Web IDL values instead of JavaScript values; this is to be understood to perform an implicit conversion to the JavaScript value before invoking these algorithms.

Call sites that are not invoked as a result of author code synchronously calling into a user agent method must take care to properly prepare to run script and prepare to run a callback before invoking these abstract operations, if they are being performed on arbitrary objects. This is necessary because the StructuredClone operation can invoke author-defined accessors as part of its final deep-cloning steps, and these accessors could call into operations that rely on the entry and incumbent concepts being properly set up.

postMessage performs StructuredCloneWithTransfer on its arguments, but is careful to do so immediately, inside the synchronous portion of its algorithm. Thus it is able to use the structured cloning algorithms without needing to prepare to run script and prepare to run a callback.

Parsing URLs can happen at a variety of times that are not the synchronous result of author code. However, the URL parser only applies StructuredClone to Blob objects, so it is not in danger of running author code with incorrect entry and incumbent values, and thus it too does not need to perform these preparation steps.

In contrast, a hypothetical API that used StructuredClone to serialize some author-supplied object periodically, directly from a task on the event loop, would need to ensure it performs the appropriate preparations before calling into the structured clone algorithms. As of this time, we know of no such APIs on the platform; usually it is simpler to perform the clone ahead of time, as a synchronous consequence of author code, like postMessage.

2.7.8 Monkey patch for Blob and FileList objects

This monkey patch will be removed in due course. See w3c/FileAPI issue 32.

Blob objects are cloneable objects.

Each Blob object's [[Clone]](targetRealm, memory) internal method must run these steps:

1. If this is closed, then throw a "DataCloneError" DOMException.

2. Return a new instance of this in targetRealm, corresponding to the same underlying data.

FileList objects are cloneable objects.

Each FileList object's [[Clone]](targetRealm, memory) internal method must run these steps:

1. Let output be a new FileList object in targetRealm.

2. For each file in this, add ? StructuredClone(file, targetRealm, memory) to the end of the list of File objects of output.

3. Return output.