XHTML

XHTML (Extensible Hypertext Markup Language) is a family of XML markup languages that mirror or extend versions of the widely used Hypertext Markup Language (HTML), the language in which web pages are written.

While HTML (prior to HTML5) was defined as an application of Standard Generalized Markup Language (SGML), a very flexible markup language framework, XHTML is an application of XML, a more restrictive subset of SGML. Because XHTML documents need to be well-formed, they can be parsed using standard XML parsers—unlike HTML, which requires a lenient HTML-specific parser.

XHTML 1.0 became a World Wide Web Consortium (W3C) Recommendation on January 26, 2000. XHTML 1.1 became a W3C Recommendation on May 31, 2001. XHTML5 is undergoing development as of September 2009, as part of the HTML5 specification.

Overview
XHTML 1.0 is "a reformulation of the three HTML 4 document types as applications of XML 1.0". The World Wide Web Consortium (W3C) also continues to maintain the HTML 4.01 Recommendation, and the specifications for HTML5 and XHTML5 are being actively developed. In the current XHTML 1.0 Recommendation document, as published and revised to August 2002, the W3C commented that, "The XHTML family is the next step in the evolution of the Internet. By migrating to XHTML today, content developers can enter the XML world with all of its attendant benefits, while still remaining confident in their content's backward and future compatibility."

However, in 2004, the Web Hypertext Application Technology Working Group (WHATWG) formed, independently of the W3C, to work on advancing ordinary HTML not based on XHTML. Most major browser vendors were unwilling to implement the features in new W3C XHTML 2.0 drafts, and felt that they didn't serve the needs of modern web development. The WHATWG eventually began working on a standard that supported both XML and non-XML serializations, HTML 5, in parallel to W3C standards such as XHTML 2. In 2007, the W3C's HTML working group voted to officially recognize HTML 5 and work on it as the next-generated HTML standard. In 2009, the W3C allowed the XHTML 2 Working Group's charter to expire, acknowledging that HTML 5 would be the sole next-generation HTML standard, including both XML and non-XML serializations.

Motivation
XHTML was developed to make HTML more extensible and increase interoperability with other data formats. HTML 4 was ostensibly an application of Standard Generalized Markup Language (SGML); however the specification for SGML was complex, and neither web browsers nor the HTML 4 Recommendation were fully conformant to it. The XML standard, approved in 1998, provided a simpler data format closer in simplicity to HTML 4. By shifting to an XML format, it was hoped HTML would become compatible with common XML tools; servers and proxies would be able to transform content, as necessary, for constrained devices such as mobile phones. By utilizing namespaces, XHTML documents could provide extensibility by including fragments from other XML-based languages such as Scalable Vector Graphics and MathML. Finally, the renewed work would provide an opportunity to divide HTML into reusable components (XHTML Modularization) and clean up untidy parts of the language.

Relationship to HTML
There are various differences between XHTML and HTML. The Document Object Model is a tree structure which represents the page internally in applications, and XHTML and HTML are two different ways of representing that in markup (serialisations). Both are less expressive than the DOM (for example, "--" may be placed in comments in the DOM, but cannot be represented in a comment in either XHTML or HTML), and generally XHTML's XML syntax is a little more expressive than HTML (for example, arbitrary namespaces are not allowed in HTML). So, firstly one source of differences is immediate: XHTML uses an XML syntax, while HTML uses a pseudo-SGML syntax (officially SGML for HTML 4 and under, but never in practice, and standardised away from SGML in HTML5). Secondly however, because the expressible contents of the DOM in syntax are slightly different, there are some changes in actual behaviour between the two models.

Firstly then, syntax differences:
 * Broadly, the XML rules require that all elements be closed, either by a separate closing tag or using self closing syntax (e.g. ), while HTML syntax permits some elements to be unclosed because either they are always empty (e.g.  ) or their end can be determined implicitly ("omissibility", e.g.  ).
 * XML is case-sensitive for element and attribute names, while HTML is not.
 * Some shorthand features in HTML are omitted in XML, such as (1) attribute minimization, where attribute values or their quotes may be omitted (e.g.  or , while XML this must be expressed as  ); (2) element minimization may be used to remove elements entirely (such as   inferred in a table if not given); and (3) the rarely used SGML syntax for element minimization ("shorttag"), which most browsers do not implement.
 * There are numerous other technical requirements surrounding namespaces and precise parsing of whitespace and certain characters and elements. The exact parsing of HTML in practice has been undefined until recently; see the HTML5 specification ([HTML5 ]) for full details, or the working summary (HTML vs. XHTML).

Secondly, in contrast to these syntactical differences which are minor, there are some behavioural differences. Most of these arise from the underlying differences in serialisation. For example:
 * Most prominently, behaviour on parse errors differ. A fatal parse error in XML (such as an incorrect tag structure) causes document processing to be aborted (a "yellow screen of death").
 * Most content requiring namespaces will not work in HTML, except the built-in support for SVG and MathML in the HTML5 parser along certain magic prefixes such as.
 * JavaScript processing is a little different, with minor changes in case sensitivity to some functions, and further precautions in XHTML to restrict processing well-formed to well-formed content. Third-party scripts, such as sidebars proving live feeds or Google's "web clips", often rely on the  method, which is not available in XHTML pages so the scripts fail on pages served as XHTML (that is, using an XML MIME type). The   property is available on XML pages, but uses the same XML parser as the whole page, so will not insert content if a non-well-formed string is passed. More positively,   in XHTML can be used to insert namespaced content to the page.
 * CSS is also applied slightly differently too, with case-sensitivity and some differences in handling of backgrounds on  and   in XHTML and HTML.

Adoption
The similarities between HTML 4.01 and XHTML 1.0 led many web sites and content management systems to adopt the initial W3C XHTML 1.0 Recommendation. To aid authors in the transition, the W3C provided guidance on how to publish XHTML 1.0 documents in an HTML-compatible manner, and serve them to browsers that were not designed for XHTML.

Such "HTML-compatible" content is sent using the HTML media type rather than the official Internet media type for XHTML. When measuring the adoption of XHTML to that of regular HTML, therefore, it is important to distinguish whether it is media type usage or actual document contents that is being compared.

Most web browsers have mature support for all of the possible XHTML media types. The notable exception is Internet Explorer by Microsoft; rather than rendering  content, a dialog box invites the user to save the content to disk instead. Both Internet Explorer 7 (released in 2006) and Internet Explorer 8 (released in March 2009) exhibit this behavior. Microsoft developer Chris Wilson explained in 2005 that IE7’s priorities were improved security and CSS support, and that proper XHTML support would be difficult to graft onto IE’s compatibility-oriented HTML parser ; however, Microsoft has added support for true XHTML in current developer previews of IE9.

As long as support is not widespread, most web developers avoid using XHTML that isn’t HTML-compatible, so advantages of XML such as namespaces, faster parsing and smaller-footprint browsers do not benefit the user.

Criticism
In the early 2000s, some web developers began to question why Web authors ever made the leap into authoring in XHTML. Others countered that the problems ascribed to the use of XHTML could mostly be attributed to two main sources: the production of invalid XHTML documents by some Web authors and the lack of support for XHTML built into Internet Explorer 6. They went on to describe the benefits of XML-based Web documents (i.e. XHTML) regarding searching, indexing and parsing as well as future-proofing the Web itself.

In October 2006, HTML inventor and W3C chair Tim Berners-Lee, introducing a major W3C effort to develop new HTML 5 and XHTML 5 specifications, posted in his blog that, "The attempt to get the world to switch to XML … all at once didn't work. The large HTML-generating public did not move … Some large communities did shift and are enjoying the fruits of well-formed systems … The plan is to charter a completely new HTML group." In the current HTML and XHTML 5 working draft, its authors say that, "special attention has been given to defining clear conformance criteria for user agents in an effort to improve interoperability … while at the same time updating the HTML specifications to address issues raised in the past few years." Ian Hickson, author of a paper criticising the improper use of XHTML in 2002, is a member of the group developing this specification and is listed as one of the co-authors of the current working draft.

Simon Pieters researched the XML-compliance of mobile browsers and concluded “the claim that XHTML would be needed for mobile devices is simply a myth”.

XHTML 1.0
December 1998 saw the publication of a W3C Working Draft entitled Reformulating HTML in XML. This introduced Voyager, the codename for a new markup language based on HTML 4 but adhering to the stricter syntax rules of XML. By February 1999 the specification had changed name to XHTML 1.0: The Extensible HyperText Markup Language, and in January 2000 it was officially adopted as a W3C Recommendation. There are three formal DTDs for XHTML 1.0, corresponding to the three different versions of HTML 4.01:
 * XHTML 1.0 Strict is the XML equivalent to strict HTML 4.01, and includes elements and attributes that have not been marked deprecated in the HTML 4.01 specification.
 * XHTML 1.0 Transitional is the XML equivalent of HTML 4.01 Transitional, and includes the presentational elements (such as,   and  ) excluded from the strict version.
 * XHTML 1.0 Frameset is the XML equivalent of HTML 4.01 Frameset, and allows for the definition of frameset documents—a common Web feature in the late 1990s.

The second edition of XHTML 1.0 became a W3C Recommendation in August 2002.

Modularization of XHTML
Modularization provides an abstract collection of components through which XHTML can be subsetted and extended. The feature is intended to help XHTML extend its reach onto emerging platforms, such as mobile devices and Web-enabled televisions. The initial draft of Modularization of XHTML became available in April 1999, and reached Recommendation status in April 2001.

The first XHTML Family Markup Languages to be developed with this technique were XHTML 1.1 and XHTML Basic 1.0. Another example is XHTML-Print (W3C Recommendation, September 2006), a language designed for printing from mobile devices to low-cost printers.

In October 2008 Modularization of XHTML was superseded by XHTML Modularization 1.1, which adds an XML Schema implementation.

XHTML 1.1—Module-based XHTML
XHTML 1.1 evolved out of the work surrounding the initial Modularization of XHTML specification. The W3C released a first draft in September 1999; Recommendation status was reached in May 2001. The modules combined within XHTML 1.1 effectively recreate XHTML 1.0 Strict, with the addition of ruby annotation elements (, ,  ,  ,   and  ) to better support East-Asian languages. Other changes include removal of the  attribute (in favour of  ), and removal of the   attribute from the   and   elements.

Although XHTML 1.1 is largely compatible with XHTML 1.0 and HTML 4, in August 2002 the HTML WG (renamed to XHTML2 WG since) issued a Working Group Note advising that it should not be transmitted with the HTML media type. With limited browser support for the alternate  media type, XHTML 1.1 proved unable to gain widespread use. In January 2009 a second edition of the document (XHTML Media Types - Second Edition, not to be confused with the XHTML 1.1 - 2nd ed) was issued, relaxing this restriction and allowing XHTML 1.1 to be served as.

XHTML 1.1 Second Edition (W3C Proposed Edited Recommendation) was issued on 7 May 2009 and rescinded on 19 May 2009. (This does not affect the text/html media type usage for XHTML 1.1 as specified in the: XHTML Media Types - Second Edition)

XHTML Basic and XHTML-MP
To support constrained devices, XHTML Basic was created by the W3C; it reached Recommendation status in December 2000. XHTML Basic 1.0 is the most restrictive version of XHTML, providing a minimal set of features that even the most limited devices can be expected to support.

The Open Mobile Alliance and its predecessor the WAP Forum released three specifications between 2001 and 2006 that extended XHTML Basic 1.0. Known as XHTML Mobile Profile or XHTML-MP, they were strongly focused on uniting the differing markup languages used on mobile handsets at the time. All provide richer form controls than XHTML Basic 1.0, along with varying levels of scripting support.

XHTML Basic 1.1 became a W3C Recommendation in July 2008, superseding XHTML-MP 1.2. XHTML Basic 1.1 is almost but not quite a subset of regular XHTML 1.1. The most notable addition over XHTML 1.1 is the  attribute—also found in XHTML-MP 1.2—which provides hints to help browsers improve form entry.

XHTML 1.2
The XHTML 2 Working Group is considering the creation of a new language based on XHTML 1.1. If XHTML 1.2 is created, it will include WAI-ARIA and  attributes to better support accessible web applications, and improved Semantic Web support through RDFa. The  attribute from XHTML Basic 1.1, along with the   attribute (for specifying frame targets) may also be present. It's important to note that the XHTML2 WG have not yet been chartered to carry out the development of XHTML1.2 and the W3C has announced that it does not intend to recharter the XHTML2 WG, this means that the XHTML1.2 proposal may not eventuate.

XHTML 2.0
Between August 2002 and July 2006 the W3C released the first eight Working Drafts of XHTML 2.0, a new version of XHTML able to make a clean break from the past by discarding the requirement of backward compatibility. This lack of compatibility with XHTML 1.x and HTML 4 caused some early controversy in the web developer community. Some parts of the language (such as the  and RDFa attributes) were subsequently split out of the specification and worked on as separate modules, partially to help make the transition from XHTML 1.x to XHTML 2.0 smoother. A ninth draft of XHTML 2.0 was expected to appear in 2009, but on July 2, 2009, the W3C decided to let the XHTML2 Working Group charter expire by that year's end, effectively halting any further development of the draft into a standard.

New features introduced by XHTML 2.0 include:
 * HTML forms will be replaced by XForms, an XML-based user input specification allowing forms to be displayed appropriately for different rendering devices.
 * HTML frames will be replaced by XFrames.
 * The DOM Events will be replaced by XML Events, which uses the XML Document Object Model.
 * A new list element type, the  element type, will be included to specifically designate a list as a navigation list. This will be useful in creating nested menus, which are currently created by a wide variety of means like nested unordered lists or nested definition lists.
 * Any element will be able to act as a hyperlink, e. g.,, similar to XLink. However, XLink itself is not compatible with XHTML due to design differences.
 * Any element will be able to reference alternative media with the  attribute, e. g.,   is the same as.
 * The  attribute of the   element has been removed: alternative text will be given in the content of the   element, much like the   element, e. g.,.
 * A single heading element will be added. The level of these headings are determined by the depth of the nesting. This allows the use of headings to be infinite, rather than limiting use to six levels deep.
 * The remaining presentational elements,   and  , still allowed in XHTML 1.x (even Strict), will be absent from XHTML 2.0. The only somewhat presentational elements remaining will be   and   for superscript and subscript respectively, because they have significant non-presentational uses and are required by certain languages. All other tags are meant to be semantic instead (e. g.   for strong or bolded text) while allowing the user agent to control the presentation of elements via CSS.
 * The addition of RDF triple with the  and   attributes to facilitate the conversion from XHTML to RDF/XML.

HTML5—Vocabulary and APIs for HTML5 and XHTML5
HTML5 initially grew independently of the W3C, through a loose group of browser manufacturers and other interested parties calling themselves the WHATWG, or Web Hypertext Application Technology Working Group. The WHATWG announced the existence of an open mailing list in June 2004, along with a website bearing the strapline “Maintaining and evolving HTML since 2004.” The key motive of the group was to create a platform for dynamic web applications; they considered XHTML 2.0 to be too document-centric, and not suitable for the creation of internet forum sites or online shops.

In April 2007, the Mozilla Foundation and Opera Software joined Apple in requesting that the newly rechartered HTML Working Group of the W3C adopt the work, under the name of HTML 5. The group resolved to do this the following month, and the First Public Working Draft of HTML 5 was issued by the W3C in January 2008. The most recent W3C Working Draft was published in June 2008.

HTML5 has both a regular  serialization and an XML serialization, which is known as XHTML5. In addition to the markup language, the specification includes a number of application programming interfaces. The Document Object Model is extended with APIs for editing, drag-and-drop, data storage and network communication.

The language is more compatible with HTML 4 and XHTML 1.x than XHTML 2.0, due to the decision to keep the existing HTML form elements and events model. It adds many new elements not found in XHTML 1.x, however, such as  and. (The XHTML 1.2 equivalent (which (X)HTML5 replaces) of these structural elements would be  and  .)

As of 2009-09-03, the latest editor’s draft includes WAI-ARIA support.

Valid XHTML documents
An XHTML document that conforms to an XHTML specification is said to be valid. Validity assures consistency in document code, which in turn eases processing, but does not necessarily ensure consistent rendering by browsers. A document can be checked for validity with the W3C Markup Validation Service. In practice, many web development programs provide code validation based on the W3C standards.

Root element
The root element of an XHTML document must be, and must contain an   attribute to associate it with the XHTML namespace. The namespace URI for XHTML is. The example tag below additionally features an  attribute to identify the document with a natural language:

DOCTYPEs
In order to validate an XHTML document, a Document Type Declaration, or DOCTYPE, may be used. A DOCTYPE declares to the browser the Document Type Definition (DTD) to which the document conforms. A Document Type Declaration should be placed before the root element.

The system identifier part of the DOCTYPE, which in these examples is the URL that begins with http://, need only point to a copy of the DTD to use, if the validator cannot locate one based on the public identifier (the other quoted string). It does not need to be the specific URL that is in these examples; in fact, authors are encouraged to use local copies of the DTD files when possible. The public identifier, however, must be character-for-character the same as in the examples.

XML declaration
A character encoding may be specified at the beginning of an XHTML document in the XML declaration when the document is served using the  MIME type. (If an XML document lacks encoding specification, an XML parser assumes that the encoding is UTF-8 or UTF-16, unless the encoding has already been determined by a higher protocol.)

For example:



The declaration may be optionally omitted because it declares as its encoding the default encoding. However, if the document instead makes use of XML 1.1 or another character encoding, a declaration is necessary. Internet Explorer prior to version 7 enters quirks mode, if it encounters an XML declaration in a document served as.

Common errors
Some of the most common errors in the usage of XHTML are:


 * Not closing empty elements (elements without closing tags in HTML4)
 * Incorrect:
 * Correct: Note that any of these is acceptable in XHTML: ,  , and  . Older HTML-only browsers interpreting it as HTML will generally accept   and.
 * Not closing non-empty elements
 * Incorrect:
 * Correct:
 * Improperly nesting elements (Note that this would also be invalid in HTML)
 * Incorrect:
 * Correct:
 * Not putting quotation marks around attribute values
 * Incorrect:
 * Incorrect:
 * Correct:
 * Correct:
 * Using the ampersand character outside of entities (Note that this would also be invalid in HTML)
 * Incorrect:
 * Correct:
 * Incorrect:
 * Correct:
 * Failing to recognize that XHTML elements and attributes are case sensitive
 * Incorrect:
 * Correct:
 * Using attribute minimization
 * Incorrect:
 * Correct:
 * Misusing CDATA, script-comments and xml-comments when embedding scripts and stylesheets.
 * This problem can be avoided altogether by putting all script and stylesheet information into separate files and referring to them as follows in the XHTML  element.


 * Note: The format, rather than the more concise  , is required for HTML compatibility when served as MIME type.
 * If an author chooses to include script or style data inline within an XHTML document, different approaches are recommended as shown in the examples below, depending whether the author intends to serve the page as  and target only fully conformant browsers, or serve the page as   and try to obtain usability in Internet Explorer 6 and other non-conformant browsers.

Backward compatibility
XHTML 1.x documents are mostly backward compatible with HTML 4 user agents when the appropriate guidelines are followed. XHTML 1.1 is essentially compatible, although the elements for ruby annotation are not part of the HTML 4 specification and thus generally ignored by HTML 4 browsers. Later XHTML 1.x modules such as those for the  attribute, RDFa and WAI-ARIA degrade gracefully in a similar manner.

XHTML 2 is significantly less compatible, although this can be mitigated to some degree through the use of scripting. (This can be simple one-liners, such as the use of “ ” to register a new HTML element within Internet Explorer, or complete JavaScript frameworks, such as the FormFaces implementation of XForms.)

Examples
The following are examples of XHTML 5, with both having the same visual output. The former one follows the HTML Compatibility Guidelines of the XHTML Media Types Note while the latter one breaks backward compatibility, but provides cleaner markup.

Example 1.

Example 2.

Notes:
 * 1) The "loadpdf" function is actually a workaround for Internet Explorer. It can be replaced by adding   within.
 * 2) The   element does not get a   attribute in the XHTML 1.0 Strict DTD. Use   instead.

Cross-compatibility of XHTML and HTML
HTML5 and XHTML 5 serializations are largely inter-compatible if adhering to the stricter XHTML5 syntax, but there are some cases in which XHTML will not work as valid HTML5 (e.g., processing instructions are deprecated in HTML, are treated as comments, and close on the first "?", whereas they are fully allowed in XML, are treated as their own type, and close on "?>").