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In some cases, requirements are given in the form of lists with conditions and corresponding requirements. In such cases, the requirements that apply to a condition are always the first set of requirements that follow the condition, even in the case of there being multiple sets of conditions for those requirements. Such cases are presented as follows:. In general, due to the Internet's architecture, a user can be distinguished from another by the user's IP address. IP addresses do not perfectly match to a user; as a user moves from device to device, or from network to network, their IP address will change; similarly, NAT routing, proxy servers, and shared computers enable packets that appear to all come from a single IP address to actually map to multiple users.

Technologies such as onion routing can be used to further anonymize requests so that requests from a single user at one node on the Internet appear to come from many disparate parts of the network. However, the IP address used for a user's requests is not the only mechanism by which a user's requests could be related to each other.

Cookies, for example, are designed specifically to enable this, and are the basis of most of the Web's session features that enable you to log into a site with which you have an account. There are other mechanisms that are more subtle. Certain characteristics of a user's system can be used to distinguish groups of users from each other; by collecting enough such information, an individual user's browser's "digital fingerprint" can be computed, which can be as good as, if not better than, an IP address in ascertaining which requests are from the same user.

Grouping requests in this manner, especially across multiple sites, can be used for both benign and even arguably positive purposes, as well as for malevolent purposes. An example of a reasonably benign purpose would be determining whether a particular person seems to prefer sites with dog illustrations as opposed to sites with cat illustrations based on how often they visit the sites in question and then automatically using the preferred illustrations on subsequent visits to participating sites.

Malevolent purposes, however, could include governments combining information such as the person's home address determined from the addresses they use when getting driving directions on one site with their apparent political affiliations determined by examining the forum sites that they participate in to determine whether the person should be prevented from voting in an election. Since the malevolent purposes can be remarkably evil, user agent implementers are encouraged to consider how to provide their users with tools to minimize leaking information that could be used to fingerprint a user.

Unfortunately, as the first paragraph in this section implies, sometimes there is great benefit to be derived from exposing the very information that can also be used for fingerprinting purposes, so it's not as easy as simply blocking all possible leaks. For instance, the ability to log into a site to post under a specific identity requires that the user's requests be identifiable as all being from the same user, more or less by definition.

More subtly, though, information such as how wide text is, which is necessary for many effects that involve drawing text onto a canvas e. In this case, by potentially exposing, via a brute force search, which fonts a user has installed, information which can vary considerably from user to user. Features in this specification which can be used to fingerprint the user are marked as this paragraph is. Other features in the platform can be used for the same purpose, though, including, though not limited to:.

The postMessage API provides a mechanism by which two sites can communicate directly. At first glance, this might appear to open a new way by which the problems described above can occur. However, in practice, multiple mechanisms exist by which two sites can communicate that predate this API: a site embedding another can send data via an iframe element's dimensions; a site can use a cross-site image request with a unique identifier known to the server to initiate a server-side data exchange; or indeed the fingerprinting techniques described above can be used by two sites to uniquely identify a visitor such that information can then be exchanged on the server side.

Fundamentally, users that do not trust a site to treat their information with respect have to avoid visiting that site at all. HTML documents consist of a tree of elements and text. Certain start tags and end tags can in certain cases be omitted and are implied by other tags. Tags have to be nested such that elements are all completely within each other, without overlapping:. This specification defines a set of elements that can be used in HTML, along with rules about the ways in which the elements can be nested.

Elements can have attributes, which control how the elements work. In the example below, there is a hyperlink , formed using the a element and its href attribute:. Otherwise, it has to be quoted using either single or double quotes. HTML user agents e. A DOM tree is an in-memory representation of a document. The markup snippet at the top of this section would be turned into the following DOM tree:.

The document element of this tree is the html element, which is the element always found in that position in HTML documents. It contains two elements, head and body , as well as a Text node between them. However, for historical reasons not all of the spaces and line breaks in the original markup appear in the DOM. In particular, all the whitespace before head start tag ends up being dropped silently, and all the whitespace after the body end tag ends up placed at the end of the body.

The head element contains a title element, which itself contains a Text node with the text "Sample page". Similarly, the body element contains an h1 element, a p element, and a comment. This DOM tree can be manipulated from scripts in the page. Scripts typically in JavaScript are small programs that can be embedded using the script element or using event handler content attributes.

For example, here is a form with a script that sets the value of the form's output element to say "Hello World":.

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Each element in the DOM tree is represented by an object, and these objects have APIs so that they can be manipulated. For instance, a link e. Since DOM trees are used as the way to represent HTML documents when they are processed and presented by implementations especially interactive implementations like Web browsers , this specification is mostly phrased in terms of DOM trees, instead of the markup described above. HTML documents represent a media-independent description of interactive content. HTML documents might be rendered to a screen, or through a speech synthesizer, or on a braille display.

To influence exactly how such rendering takes place, authors can use a styling language such as CSS. For more details on how to use HTML, authors are encouraged to consult tutorials and guides. Some of the examples included in this specification might also be of use, but the novice author is cautioned that this specification, by necessity, defines the language with a level of detail that might be difficult to understand at first. When HTML is used to create interactive sites, care needs to be taken to avoid introducing vulnerabilities through which attackers can compromise the integrity of the site itself or of the site's users.

A comprehensive study of this matter is beyond the scope of this document, and authors are strongly encouraged to study the matter in more detail. However, this section attempts to provide a quick introduction to some common pitfalls in HTML application development. The security model of the Web is based on the concept of "origins", and correspondingly many of the potential attacks on the Web involve cross-origin actions.

When accepting untrusted input, e. Failing to do this can allow a hostile user to perform a variety of attacks, ranging from the potentially benign, such as providing bogus user information like a negative age, to the serious, such as running scripts every time a user looks at a page that includes the information, potentially propagating the attack in the process, to the catastrophic, such as deleting all data in the server.

When writing filters to validate user input, it is imperative that filters always be safelist-based, allowing known-safe constructs and disallowing all other input. Blocklist-based filters that disallow known-bad inputs and allow everything else are not secure, as not everything that is bad is yet known for example, because it might be invented in the future. For example, suppose a page looked at its URL's query string to determine what to display, and the site then redirected the user to that page to display a message, as in:. If the message was just displayed to the user without escaping, a hostile attacker could then craft a URL that contained a script element:.

If the attacker then convinced a victim user to visit this page, a script of the attacker's choosing would run on the page. Such a script could do any number of hostile actions, limited only by what the site offers: if the site is an e-commerce shop, for instance, such a script could cause the user to unknowingly make arbitrarily many unwanted purchases.

There are many constructs that can be used to try to trick a site into executing code. Here are some that authors are encouraged to consider when writing safelist filters:. If a site allows a user to make form submissions with user-specific side-effects, for example posting messages on a forum under the user's name, making purchases, or applying for a passport, it is important to verify that the request was made by the user intentionally, rather than by another site tricking the user into making the request unknowingly.

A page that provides users with an interface to perform actions that the user might not wish to perform needs to be designed so as to avoid the possibility that users can be tricked into activating the interface.


One way that a user could be so tricked is if a hostile site places the victim site in a small iframe and then convinces the user to click, for instance by having the user play a reaction game. Once the user is playing the game, the hostile site can quickly position the iframe under the mouse cursor just as the user is about to click, thus tricking the user into clicking the victim site's interface. To avoid this, sites that do not expect to be used in frames are encouraged to only enable their interface if they detect that they are not in a frame e.

Scripts in HTML have "run-to-completion" semantics, meaning that the browser will generally run the script uninterrupted before doing anything else, such as firing further events or continuing to parse the document. On the other hand, parsing of HTML files happens incrementally, meaning that the parser can pause at any point to let scripts run. This is generally a good thing, but it does mean that authors need to be careful to avoid hooking event handlers after the events could have possibly fired.

There are two techniques for doing this reliably: use event handler content attributes , or create the element and add the event handlers in the same script. The latter is safe because, as mentioned earlier, scripts are run to completion before further events can fire. One way this could manifest itself is with img elements and the load event. The event could fire as soon as the element has been parsed, especially if the image has already been cached which is common.

Here, the author uses the onload handler on an img element to catch the load event:. If the element is being added by script, then so long as the event handlers are added in the same script, the event will still not be missed:. However, if the author first created the img element and then in a separate script added the event listeners, there's a chance that the load event would be fired in between, leading it to be missed:.

Authors are encouraged to make use of conformance checkers also known as validators to catch common mistakes. Unlike previous versions of the HTML specification, this specification defines in some detail the required processing for invalid documents as well as valid documents. However, even though the processing of invalid content is in most cases well-defined, conformance requirements for documents are still important: in practice, interoperability the situation in which all implementations process particular content in a reliable and identical or equivalent way is not the only goal of document conformance requirements.

This section details some of the more common reasons for still distinguishing between a conforming document and one with errors. The majority of presentational features from previous versions of HTML are no longer allowed. Presentational markup in general has been found to have a number of problems:. While it is possible to use presentational markup in a way that provides users of assistive technologies ATs with an acceptable experience e. Furthermore, even using such techniques doesn't help make pages accessible for non-AT non-graphical users, such as users of text-mode browsers.

Using media-independent markup, on the other hand, provides an easy way for documents to be authored in such a way that they work for more users e. It is significantly easier to maintain a site written in such a way that the markup is style-independent. Presentational markup tends to be much more redundant, and thus results in larger document sizes. For those reasons, presentational markup has been removed from HTML in this version.

The only remaining presentational markup features in HTML are the style attribute and the style element. Use of the style attribute is somewhat discouraged in production environments, but it can be useful for rapid prototyping where its rules can be directly moved into a separate style sheet later and for providing specific styles in unusual cases where a separate style sheet would be inconvenient. Similarly, the style element can be useful in syndication or for page-specific styles, but in general an external style sheet is likely to be more convenient when the styles apply to multiple pages.

It is also worth noting that some elements that were previously presentational have been redefined in this specification to be media-independent: b , i , hr , s , small , and u. Certain invalid syntax constructs, when parsed, result in DOM trees that are highly unintuitive. For example, the following markup fragment results in a DOM with an hr element that is an earlier sibling of the corresponding table element:.

To allow user agents to be used in controlled environments without having to implement the more bizarre and convoluted error handling rules, user agents are permitted to fail whenever encountering a parse error. To avoid interoperability problems with such user agents, any syntax resulting in such behavior is considered invalid. Most syntax constructs that require such handling are considered invalid. Comments containing two consecutive hyphens, or ending with a hyphen, are exceptions that are allowed in the HTML syntax.

Certain syntax constructs can result in disproportionately poor performance. To discourage the use of such constructs, they are typically made non-conforming. For example, the following markup results in poor performance, since all the unclosed i elements have to be reconstructed in each paragraph, resulting in progressively more elements in each paragraph:.

There are syntax constructs that, for historical reasons, are relatively fragile. To help reduce the number of users who accidentally run into such problems, they are made non-conforming. For example, the parsing of certain named character references in attributes happens even with the closing semicolon being omitted. It is safe to include an ampersand followed by letters that do not form a named character reference, but if the letters are changed to a string that does form a named character reference, they will be interpreted as that character instead.

In the following fragment, however, the attribute's value is actually "? To avoid this problem, all named character references are required to end with a semicolon, and uses of named character references without a semicolon are flagged as errors. Certain syntax constructs are known to cause especially subtle or serious problems in legacy user agents, and are therefore marked as non-conforming to help authors avoid them.

In certain legacy user agents, it is sometimes treated as a quote character. Another example of this is the DOCTYPE, which is required to trigger no-quirks mode , because the behavior of legacy user agents in quirks mode is often largely undocumented. For example, the restriction on using UTF-7 exists purely to avoid authors falling prey to a known cross-site-scripting attack using UTF Markup where the author's intent is very unclear is often made non-conforming.

Correcting these errors early makes later maintenance easier. For example, it is unclear whether the author intended the following to be an h1 heading or an h2 heading:. When a user makes a simple typo, it is helpful if the error can be caught early, as this can save the author a lot of debugging time. This specification therefore usually considers it an error to use element names, attribute names, and so forth, that do not match the names defined in this specification. In order to allow the language syntax to be extended in the future, certain otherwise harmless features are disallowed.

For example, "attributes" in end tags are ignored currently, but they are invalid, in case a future change to the language makes use of that syntax feature without conflicting with already-deployed and valid! Some authors find it helpful to be in the practice of always quoting all attributes and always including all optional tags, preferring the consistency derived from such custom over the minor benefits of terseness afforded by making use of the flexibility of the HTML syntax.

To aid such authors, conformance checkers can provide modes of operation wherein such conventions are enforced. Beyond the syntax of the language, this specification also places restrictions on how elements and attributes can be specified. These restrictions are present for similar reasons:. To avoid misuse of elements with defined meanings, content models are defined that restrict how elements can be nested when such nestings would be of dubious value. For example, this specification disallows nesting a section element inside a kbd element, since it is highly unlikely for an author to indicate that an entire section should be keyed in.

Similarly, to draw the author's attention to mistakes in the use of elements, clear contradictions in the semantics expressed are also considered conformance errors. In the fragments below, for example, the semantics are nonsensical: a separator cannot simultaneously be a cell, nor can a radio button be a progress bar. Another example is the restrictions on the content models of the ul element, which only allows li element children.

Lists by definition consist just of zero or more list items, so if a ul element contains something other than an li element, it's not clear what was meant. Certain elements have default styles or behaviors that make certain combinations likely to lead to confusion. Where these have equivalent alternatives without this problem, the confusing combinations are disallowed. For example, div elements are rendered as block boxes , and span elements as inline boxes. Putting a block box in an inline box is unnecessarily confusing; since either nesting just div elements, or nesting just span elements, or nesting span elements inside div elements all serve the same purpose as nesting a div element in a span element, but only the latter involves a block box in an inline box , the latter combination is disallowed.

Another example would be the way interactive content cannot be nested. For example, a button element cannot contain a textarea element. This is because the default behavior of such nesting interactive elements would be highly confusing to users. Instead of nesting these elements, they can be placed side by side. For example, setting the disabled attribute to the value " false " is disallowed, because despite the appearance of meaning that the element is enabled, it in fact means that the element is disabled what matters for implementations is the presence of the attribute, not its value.

For example, the area element's shape attribute, despite accepting both circ and circle values in practice as synonyms, disallows the use of the circ value, so as to simplify tutorials and other learning aids. There would be no benefit to allowing both, but it would cause extra confusion when teaching the language.

Certain elements are parsed in somewhat eccentric ways typically for historical reasons , and their content model restrictions are intended to avoid exposing the author to these issues. For example, a form element isn't allowed inside phrasing content , because when parsed as HTML, a form element's start tag will imply a p element's end tag. Thus, the following markup results in two paragraphs , not one:. This is why, for instance, it is non-conforming to have two id attributes with the same value.

Duplicate IDs lead to the wrong element being selected, with sometimes disastrous effects whose cause is hard to determine. Some constructs are disallowed because historically they have been the cause of a lot of wasted authoring time, and by encouraging authors to avoid making them, authors can save time in future efforts.

For example, a script element's src attribute causes the element's contents to be ignored.

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However, this isn't obvious, especially if the element's contents appear to be executable script — which can lead to authors spending a lot of time trying to debug the inline script without realizing that it is not executing. To reduce this problem, this specification makes it non-conforming to have executable script in a script element when the src attribute is present. This means that authors who are validating their documents are less likely to waste time with this kind of mistake.

Though this practice is discouraged in general due to the myriad of subtle complications involved especially when involving scripting, styling, or any kind of automated serialization , this specification has a few restrictions intended to at least somewhat mitigate the difficulties. For example, there are somewhat complicated rules surrounding the lang and xml:lang attributes intended to keep the two synchronized. Another example would be the restrictions on the values of xmlns attributes in the HTML serialization, which are intended to ensure that elements in conforming documents end up in the same namespaces whether processed as HTML or XML.

As with the restrictions on the syntax intended to allow for new syntax in future revisions of the language, some restrictions on the content models of elements and values of attributes are intended to allow for future expansion of the HTML vocabulary. For example, requiring that attributes that take media query lists use only valid media query lists reinforces the importance of following the conformance rules of that specification. Topics addressed include use of the terms 'character', 'encoding' and 'string', a reference processing model, choice and identification of character encodings, character escaping, and string indexing.

Unicode Security Considerations [UTR36] Because Unicode contains such a large number of characters and incorporates the varied writing systems of the world, incorrect usage can expose programs or systems to possible security attacks. This is especially important as more and more products are internationalized. This document describes some of the security considerations that programmers, system analysts, standards developers, and users should take into account, and provides specific recommendations to reduce the risk of problems. Following these guidelines will make content accessible to a wider range of people with disabilities, including blindness and low vision, deafness and hearing loss, learning disabilities, cognitive limitations, limited movement, speech disabilities, photosensitivity and combinations of these.

Following these guidelines will also often make your Web content more usable to users in general. An authoring tool that conforms to these guidelines will promote accessibility by providing an accessible user interface to authors with disabilities as well as by enabling, supporting, and promoting the production of accessible Web content by all authors. User agents include browsers and other types of software that retrieve and render Web content.

A user agent that conforms to these guidelines will promote accessibility through its own user interface and through other internal facilities, including its ability to communicate with other technologies especially assistive technologies. Furthermore, all users, not just users with disabilities, should find conforming user agents to be more usable. When these are ambiguous they are qualified as object properties and CSS properties respectively. 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 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.

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. To run steps in parallel means those steps are to be run, one after another, at the same time as other logic in the standard e. This standard does not define the precise mechanism by which this is achieved, be it time-sharing cooperative multitasking, fibers, threads, processes, using different hyperthreads, cores, CPUs, machines, etc.

By contrast, an operation that is to run immediately must interrupt the currently running task, run itself, and then resume the previously running task. For guidance on writing specifications that leverage parallelism, see Dealing with the event loop from other specifications. To avoid race conditions between different in parallel algorithms that operate on the same data, a parallel queue can be used. A parallel queue represents a queue of algorithm steps that must be run in series. A parallel queue has an algorithm queue a queue , initially empty.

To enqueue steps to a parallel queue , enqueue the algorithm steps to the parallel queue 's algorithm queue. To start a new parallel queue , run the following steps:. Let parallelQueue be a new parallel queue. Run the following steps in parallel :. Let steps be the result of dequeueing from parallelQueue 's algorithm queue.

If steps is not nothing, then run steps. Assert: running steps did not throw an exception, as steps running in parallel are not allowed to throw. Implementations are not expected to implement this as a continuously running loop. Algorithms in standards are to be easy to understand and are not necessarily great for battery life or performance. Return parallelQueue. Steps running in parallel can themselves run other steps in in parallel.

Imagine a standard defined nameList a list , along with a method to add a name to nameList , unless nameList already contains name , in which case it rejects. Let p be a new promise. If nameList contains name , reject p with a TypeError and abort these steps. Do some potentially lengthy work. Append name to nameList. Resolve p with undefined. Return p. Two invocations of the above could run simultaneously, meaning name isn't in nameList during step 2.

Parallel queues solve this. The standard would let nameListQueue be the result of starting a new parallel queue , then:. Enqueue the following steps to nameListQueue :. The steps would now queue and the race is avoided. The specification uses the term supported when referring to whether a user agent has an implementation capable of decoding the semantics of an external resource.

A format or type is said to be supported if the implementation can process an external resource of that format or type without critical aspects of the resource being ignored. Whether a specific resource is supported can depend on what features of the resource's format are in use. For example, a PNG image would be considered to be in a supported format if its pixel data could be decoded and rendered, even if, unbeknownst to the implementation, the image also contained animation data.

An MPEG-4 video file would not be considered to be in a supported format if the compression format used was not supported, even if the implementation could determine the dimensions of the movie from the file's metadata. What some specifications, in particular the HTTP specifications, refer to as a representation is referred to in this specification as a resource. A resource's critical subresources are those that the resource needs to have available to be correctly processed. Which resources are considered critical or not is defined by the specification that defines the resource's format.

For CSS style sheets , we tentatively define here that their critical subresources are other style sheets imported via import rules, including those indirectly imported by other imported style sheets. This definition is not fully interoperable; furthermore, some user agents seem to count resources like background images or web fonts as critical subresources. 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. 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. 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 's browsing context is non-null. A node becomes browsing-context connected when the insertion steps are invoked with it as the argument and it is now browsing-context connected. A node becomes browsing-context disconnected either when the removing steps are invoked with it as the argument and it is now no longer browsing-context connected , or when its shadow-including root 's browsing context becomes null.

An IDL attribute is said to be getting when its value is being retrieved e. 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. 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. 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. 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. A character encoding , or just encoding where that is not ambiguous, is a defined way to convert between byte streams and Unicode strings, as defined in Encoding.

An encoding has an encoding name and one or more encoding labels , referred to as the encoding's name and labels in the Encoding standard. Since support for encodings that are not defined in Encoding is prohibited, UTF encodings are the only encodings that this specification needs to treat as not being ASCII-compatible encodings. This specification describes the conformance criteria for user agents relevant to implementers and documents relevant to authors and authoring tool implementers.

Conforming documents are those that comply with all the conformance criteria for documents. For readability, some of these conformance requirements are phrased as conformance requirements on authors; such requirements are implicitly requirements on documents: by definition, all documents are assumed to have had an author.

In some cases, that author may itself be a user agent — such user agents are subject to additional rules, as explained below. For example, if a requirement states that "authors must not use the foobar element", it would imply that documents are not allowed to contain elements named foobar.

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

Web browsers 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.

User agents that support scripting must also be conforming implementations of the IDL fragments in this specification, as described in Web IDL. Unless explicitly stated, specifications that override the semantics of HTML elements do not override the requirements on DOM objects representing those elements. 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. 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. 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.

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 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.

The term "HTML validator" can be used to refer to a conformance checker that itself conforms to the applicable requirements of this specification. 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. A conformance checker must check for the first two.

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

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Authoring tools are exempt from the strict requirements of using elements only for their specified purpose, but only to the extent that authoring tools are not yet able to determine author intent. However, authoring tools must not automatically misuse elements or encourage their users to do so. For example, it is not conforming to use an address element for arbitrary contact information; that element can only be used for marking up contact information for its nearest article or body element ancestor.

However, since an authoring tool is likely unable to determine the difference, an authoring tool is exempt from that requirement. This does not mean, though, that authoring tools can use address elements for any block of italics text for instance ; it just means that the authoring tool doesn't have to verify that when the user uses a tool for inserting contact information for an article element, that the user really is doing that and not inserting something else instead.

In terms of conformance checking, an editor has to output documents that conform to the same extent that a conformance checker will verify. When an authoring tool is used to edit a non-conforming document, it may preserve the conformance errors in sections of the document that were not edited during the editing session i. However, an authoring tool must not claim that the output is conformant if errors have been so preserved.

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. 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.

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.

The Unicode character set is used to represent textual data, and Encoding defines requirements around character encodings. This specification introduces terminology based on the terms defined in those specifications, as described earlier. Data mining tools and other user agents that perform operations on content without running scripts, evaluating CSS or XPath expressions, or otherwise exposing the resulting DOM to arbitrary content, may "support namespaces" by just asserting that their DOM node analogues are in certain namespaces, without actually exposing the namespace strings.

For instance, the colon has no special meaning in HTML element names. The Name production is defined in XML. The term throw in this specification is used as defined in Web IDL. When this specification requires a user agent to create a Date object representing a particular time which could be the special value Not-a-Number , the milliseconds component of that time, if any, must be truncated to an integer, and the time value of the newly created Date object must represent the resulting truncated time.

For instance, given the time millionths of a second after UTC on January 1st , i. If the given time is NaN, then the result is a Date object that represents a time value NaN indicating that the object does not represent a specific instant of time. Some parts of the language described by this specification only support JavaScript as the underlying scripting language. The following terms are defined in the JavaScript specification and used in this specification:. User agents that support JavaScript must also implement the import. User agents that support JavaScript must also implement the BigInt proposal.

This specification sometimes uses the term name to refer to the event's type ; as in, "an event named click " or "if the event name is keypress ". The terms "name" and "type" for events are synonymous. User agents are encouraged to implement the features described in execCommand. Implementations must support Media Queries. While support for CSS as a whole is not required of implementations of this specification though it is encouraged, at least for Web browsers , some features are defined in terms of specific CSS requirements. When this specification requires that something be parsed according to a particular CSS grammar , the relevant algorithm in CSS Syntax must be followed, including error handling rules.

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

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If color is 'currentcolor' , then:. If element is not given, then set color to opaque black. Otherwise, set color to the computed value of the 'color' property of element. Return color. The terms intrinsic width and intrinsic height refer to the width dimension and the height dimension, respectively, of intrinsic dimensions. The following features are defined in CSS Overflow.

The algorithm to update animations and send events is defined in Web Animations. Implementations may support WebVTT as a text track format for subtitles, captions, metadata, etc. While support for MathML as a whole is not required by this specification though it is encouraged, at least for Web browsers , certain features depend upon small parts of MathML being implemented. While support for SVG as a whole is not required by this specification though it is encouraged, at least for Web browsers , certain features depend upon parts of SVG being implemented.

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

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

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

When 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. 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. Implementations of XPath 1. 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. 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. If the QName has no prefix and the principal node type of the axis is element, then the default element namespace is used. The default element namespace is a member of the context for the XPath expression.

This is equivalent to adding the default element namespace feature of XPath 2. 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. This change is a willful violation of the XPath 1. XSLT 1.

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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.

Processors that serialize the output are unaffected. 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. 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. This document defines the following policy-controlled features :. 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. 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.

A number of attributes are boolean attributes. The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value. If the attribute is present, its value must either be the empty string or a value that is an ASCII case-insensitive match for the attribute's canonical name, with no leading or trailing whitespace.

The values "true" and "false" are not allowed on boolean attributes. To represent a false value, the attribute has to be omitted altogether. Here is an example of a checkbox that is checked and disabled. The checked and disabled attributes are the boolean attributes. 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, 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 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. Let input be the string being parsed. Let position be a pointer into input , initially pointing at the start of the string.

Let sign have the value "positive". If position is past the end of input , return an error. 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. If sign is "positive", return value , otherwise return the result of subtracting value from zero.

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. This algorithm will return either zero, a positive integer, or an error. Let value be the result of parsing input using the rules for parsing integers.

If value is an error, return an error. If value is less than zero, return an error. Return value. If there is no E, then the exponent is treated as zero. 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.