XHTML

Infobox file format
name = XHTML
extension = .xhtml, .xht, .html, .htm
mime = application/xhtml+xml
owner = World Wide Web Consortium
creatorcode =
genre = Markup language


containerfor =
containedby =
extendedfrom = XML, HTML
extendedto =
standard = [http://www.w3.org/TR/xhtml1/ 1.0 (Recommendation)] ,
[http://www.w3.org/TR/2001/REC-xhtml11-20010531/ 1.1 (Recommendation)] ,
[http://www.w3.org/TR/xhtml11/ 1.1 SE (Working Draft)] ,
[http://www.w3.org/TR/html5/ 5 (Working Draft)] ,
[http://www.w3.org/TR/xhtml2/ 2.0 (Working Draft)]

The "Extensible Hypertext Markup Language", or XHTML, is a markup language that has the same depth of expression as HTML, but also conforms to XML syntax.

While HTML is an application of Standard Generalized Markup Language (SGML), a very flexible markup language, XHTML is an application of XML, a more restrictive subset of SGML. Because they need to be well-formed, true XHTML documents allow for automated processing to be performed using standard XML tools—unlike HTML, which requires a relatively complex, lenient, and generally custom parser. XHTML can be thought of as the intersection of HTML and XML in many respects, since it is a reformulation of HTML in XML. 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.

Overview

XHTML is "a reformulation of the three HTML 4 document types as applications of XML 1.0".http://www.w3.org/TR/xhtml1/#xhtml 'What is XHTML?' section in "XHTML™ 1.0 The Extensible HyperText Markup Language (Second Edition)" A Reformulation of HTML 4 in XML 1.0 "W3C Recommendation 26 January 2000, revised 1 August 2002"] The 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 comments 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."

Motivation

According to the W3C XHTML 1.0 recommendation document, with which the standards body introduced XHTML to the web community in January 2000, HTML had originally been designed for authoring relatively simple, technical documents. It quickly outgrew this original purpose, leading to the rapid invention of new elements and adaptations to specialized uses, which in turn led to interoperability problems on different platforms.

XML, as a restricted subset of SGML, preserves most of SGML's power but removes much of its complexity. In doing so, it reduces both the costs and the complexity of producing suitable authoring software.

XHTML combines the benefits of XML with those of HTML. For example, to express ideas using new markup, the XHTML family accommodates extensions as described in the XHTML Modularization specification. XHTML also maximizes interoperability: servers, proxies, and user agents can transform content into whatever format is most appropriate for various requesting agents, such as mobile users with limited capabilities, robotic agents that are mainly interested in indexable text, or home users who wish to make the most of broadband or interactive multimedia.

In 2000, the W3C concluded 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."cite web
url=http://www.w3.org/TR/xhtml1/#why
title=XHTML™ 1.0 The Extensible HyperText Markup Language (Second Edition)A Reformulation of HTML 4 in XML 1.0W3C Recommendation
accessdate=2008-10-10
publisher=The World Wide Web Consortium
date= 26 January 2000, revised 1 August 2002 ]

Relationship to HTML

HTML is the antecedent technology to XHTML. The changes from HTML to first-generation XHTML 1.0 are minor and are mainly to achieve conformance with XML. The most important change is the requirement that the document must be well-formed and that all elements must be explicitly closed as required in XML. [ [http://www.w3.org/TR/xhtml1/#h-4.1 Documents must be well-formed - W3C ] ] In XML, all element and attribute names are case-sensitive, so the XHTML approach has been to define all tag names to be lowercase.

This contrasts with some earlier established traditions which began around the time of HTML 2.0, when many used uppercase tags. In XHTML, all attribute values must be enclosed by quotes; either single (') or double (") quotes may be used. In contrast, this was sometimes optional in SGML-based HTML, where numeric or boolean attributes can omit quotes.Fact|date=May 2008 All elements must also be explicitly closed, including empty (aka singleton) elements such as img and br. This can be done by adding a closing slash to the start tag, "e.g.", <img /> and <br />. Attribute minimization (e.g., <option selected>) is also prohibited, as the attribute selected contains no explicit value; instead this would be written as <option selected="selected">.

HTML elements which are optional in the content model will not appear in the DOM tree unless they are explicitly specified. For example, an XHTML page "must" have a <body> element, and a table will not have a <tbody> element unless the author specifies one.

The XHTML 1.0 recommendation devotes a section to differences between HTML and XHTML. [ [http://www.w3.org/TR/xhtml1/#diffs XHTML 1.0: The Extensible HyperText Markup Language (Second Edition) ] ] . The WHATWG wiki similarly considers differences that arise with the use of (X)HTML5. [ [http://wiki.whatwg.org/wiki/HTML_vs._XHTML Html Vs. Xhtml - Whatwg Wiki ] ] .

Because XHTML and HTML are closely related technologies, sometimes they are written about and documented in parallel. In such circumstances, some authors conflate the two names by using a parenthetical notation, such as (X)HTML. This indicates that the documentation and principles can be considered to apply generally to both standards.For examples of this, see: http://jigsaw.w3.org/css-validator/ http://www.adobe.com/devnet/dreamweaver/articles/css_best_practices_pt2.html http://www.ukoln.ac.uk/web-focus/papers/www2003/poster-1/dcmeta.doc]

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.cite web
url=http://www.w3.org/TR/xhtml1/#guidelines
title=XHTML 1.0 Specification, Appendix C: HTML Compatibility Guidelines
publisher=World Wide Web Consortium
date=2000-01-26
accessdate=2007-06-16] cite web
url=http://www.w3.org/TR/xhtml-media-types
title=XHTML Media Types, W3C Note
publisher=World Wide Web Consortium
date=2002-08-01
accessdate=2008-06-12]

Such "HTML-compatible" content is sent using the HTML media type (text/html) rather than the official Internet media type for XHTML (application/xhtml+xml). 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 supportEarly implementations (such as Mozilla 0.7 and Opera 6.0, both released in 2001) do not incrementally render XHTML as it is received over the network, giving a degraded user experience; see the [http://www.mozilla.org/docs/web-developer/faq.html#xhtmldiff Mozilla Web Author FAQ] . Later browsers such as Opera 9.0, Safari 3.0 and Firefox 3.0 do not have this issue.] for all of the possible XHTML media types. [ [http://www.w3.org/People/mimasa/test/xhtml/media-types/results XHTML Media Type Tests – Results] ] The notable exception is Internet Explorer by Microsoft; rather than rendering application/xhtml+xml content, a dialog box invites the user to save the content to disk instead. Both Internet Explorer 7 (released in 2006) and the second beta version of Internet Explorer 8 (released in August 2008) exhibit this behavior, and it is unclear whether this will be resolved in a future release. As long as this remains the case, most web developers avoid using XHTML that isn’t HTML-compatiblecite web
url=http://www.goer.org/Markup/TheXPhiles/
title=List of XHTML Sites (the X-Philes)
accessdate=2008-08-26] , so advantages of XML such as namespaces, faster parsing and smaller-footprint browsers do not benefit the user. 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.cite web
url=http://blogs.msdn.com/ie/archive/2005/09/15/467901.aspx
title=The prolog, strict mode, and XHTML in IE
author=Chris Wilson
date=2005-09-15
accessdate=2007-06-16
quote=…If we tried to support real XHTML in IE 7 we would have ended up using our existing HTML parser (which is focused on compatibility) and hacking in XML constructs. It is highly unlikely we could support XHTML well in this way; in particular, we would certainly not detect a few error cases here or there, and we would silently support invalid cases. This would, of course, cause compatibility problems based on parser error handling in the future, which XML is explicitly trying to avoid; we don’t want to cause another mess like the one with current HTML error handling (rooted in compatibility with earlier browsers – you can blame me for that personally somewhat, but not IE). I would much rather take the time to implement XHTML properly after IE 7, and have it be truly interoperable…]

In the early 2000s, some web developers began to question why Web authors ever made the leap into authoring in XHTML.cite web
url=http://www.hixie.ch/advocacy/xhtml
title=Sending XHTML as text/html Considered Harmful
author=Ian Hickson, a former developer of the Opera browser and cofounder of the WHATWG
date=2002-09-08
accessdate=2007-06-16] cite web
url=http://annevankesteren.nl/2004/06/invalid-html
title=XHTML is invalid HTML
author=Anne van Kesteren, a developer of the Opera browser
date=2004-06-13
accessdate=2007-06-16] cite web
url=http://webkit.org/blog/?p=68
title=Understanding HTML, XML and XHTML
author=Maciej Stachowiak, a developer of Apple's Safari browser
date=2006-09-20
accessdate=2007-06-16] 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 IE6. [cite web |url=http://h3h.net/2005/12/xhtml-harmful-to-feelings/
title=Sending XHTML as text/html Considered Harmful to Feelings
author=Brad Fults
date=2005-12-21
accessdate=2008-09-13
quote=There are not nearly as many disadvantages (if any) to sending XHTML as text/html as [Ian Hickson] claims and the advantages I mentioned above make it well worth using in my humble opinion. There are some subtle footnotes and parentheticals [in Hickson's article] indicating that the harmfulness only applies to authors that don’t know the pitfalls of this practice, but much like the “Do not eat” label on the little packets of silica gel, Ian’s advisory seems to be common sense and not worth mentioning to any author who actually knows what XHTML is and how to write it.] [cite web
url=http://www.layouts4free.com/articles/view-article/49/3/0/
title=The case for XHTML
author=Paul McDonald
date=2007-06-30
accessdate=2008-09-13
quote=Some people say XHTML on the web has failed, but I say it is our biggest success in the fight for Web Standards....

XHTML is a good thing for the web, though, and it's a shame that people are trying to make a case against it. To prove this, I'll flesh out the myth for you and then show you why XHTML is the best thing since sliced bread, when it comes to our fight for Web Standards....

So to conclude, sending XHTML as text/html causes no damage or harm anywhere today, as long as your XHTML does validate. And if you want Web Standards to become more and more wide-spread, stick to using XHTML and validate your pages.] 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 XHTML 5 and HTML 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." [cite web
url=http://dig.csail.mit.edu/breadcrumbs/node/166
title=Reinventing HTML
author=Tim Berners-Lee
date=2006-10-27
accessdate=2007-06-16
quote= Some things are clearer with hindsight of several years. It is necessary to evolve HTML incrementally. The attempt to get the world to switch to XML, including quotes around attribute values and slashes in empty tags and namespaces all at once didn't work. The large HTML-generating public did not move, largely because the browsers didn't complain. Some large communities did shift and are enjoying the fruits of well-formed systems, but not all. It is important to maintain HTML incrementally, as well as continuing a transition to well-formed world, and developing more power in that world.

The plan is to charter a completely new HTML group. Unlike the previous one, this one will be chartered to do incremental improvements to HTML, as also in parallel xHTML. It will have a different chair and staff contact. It will work on HTML and xHTML together. We have strong support for this group, from many people we have talked to, including browser makers. ] 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 the important paper criticising the 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.cite web
url=http://www.w3.org/TR/2008/WD-html5-20080610/
title=HTML 5: A vocabulary and associated APIs for HTML and XHTML - W3C Working Draft 10 June 2008
author=Ian Hickson and David Hyatt
date=2008-06-10
accessdate=2008-09-13]

Versions of XHTML

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.cite web
url=http://www.w3.org/TR/2000/REC-xhtml1-20000126
title=XHTML™ 1.0: The Extensible HyperText Markup Language, W3C Recommendation 26 January 2000
publisher=World Wide Web Consortium
date=2000-01-26
accessdate=2008-07-19] 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 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 equivalent of HTML 4.01 Transitional, and includes the presentational elements (such as center, font and strike) excluded from the strict version.
* XHTML 1.0 Frameset is the 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.cite web
url=http://www.w3.org/TR/2002/REC-xhtml1-20020801
title= XHTML™ 1.0 The Extensible HyperText Markup Language (Second Edition)
publisher=World Wide Web Consortium
date=2002-08-01
accessdate=2008-07-19]

Modularization of XHTML

The initial draft of "Modularization of XHTML" became available in April 1999, and reached Recommendation status in April 2001.cite web
url=http://www.w3.org/TR/2001/REC-xhtml-modularization-20010410
title=Modularization of XHTML™, W3C Recommendation 10 April 2001
publisher=World Wide Web Consortium
date=2001-04-10
accessdate=2008-07-19] 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 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.cite web
url=http://www.w3.org/TR/2006/REC-xhtml-print-20060920/
title=XHTML-Print, W3C Recommendation 20 September 2006
publisher=World Wide Web Consortium
date=2006-09-20
accessdate=2008-07-19]

In October 2008 "Modularization of XHTML" was superseded by "XHTML Modularization 1.1", which adds an XML Schema implementation.cite web
url=http://www.w3.org/TR/2008/REC-xhtml-modularization-20081008
title=XHTML™ Modularization 1.1, W3C Recommendation 8 October 2008
publisher=World Wide Web Consortium
date=2008-10-08
accessdate=2008-10-08]

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.cite web
url=http://www.w3.org/TR/2001/REC-xhtml11-20010531
title=XHTML™ 1.1 - Module-based XHTML, W3C Recommendation 31 May 2001
publisher=World Wide Web Consortium
date=2001-05-31
accessdate=2008-07-19] The modules combined within XHTML 1.1 effectively recreate XHTML 1.0 Strict, with the addition of ruby annotation elements (ruby, rbc, rtc, rb, rt and rp) to better support East-Asian languages. Other changes include removal of the lang attribute (in favour of xml:lang), and removal of the name attribute from the a and map elements.

Although XHTML 1.1 is largely compatible with XHTML 1.0 and HTML 4, in August 2002 the W3C issued a formal Note advising that it should not be transmitted with the HTML media type.cite web
url=http://www.w3.org/TR/2002/NOTE-xhtml-media-types-20020801
title=XHTML Media Types, W3C Note 1 August 2002
publisher=World Wide Web Consortium
date=2002-08-01
accessdate=2008-07-19] With limited browser support for the alternate application/xhtml+xml media type, XHTML 1.1 has so far proven unable to gain widespread use.

XHTML 1.1 Second Edition is expected in the fourth quarter of 2008.cite web
url=http://www.w3.org/TR/2007/WD-xhtml11-20070216
title=XHTML™ 1.1 - Module-based XHTML - Second Edition, W3C Working Draft 16 February 2007
publisher=World Wide Web Consortium
date=2007-02-16
accessdate=2008-07-19]

XHTML Basic and XHTML-MP

XHTML Basic is an XML-based structured markup language primarily used for simple (mainly "handheld") user agents, typically mobile devices, created by the W3C; it reached Recommendation status in December 2000.cite web
url=http://www.w3.org/TR/2000/REC-xhtml-basic-20001219
title=XHTML™ Basic, W3C Recommendation 19 December 2000
publisher=World Wide Web Consortium
date=2000-12-19
accessdate=2008-07-19] XHTML Basic is suited for mobile phones, PDAs, pagers, and settop boxes. It is the most restrictive version of XHTML, providing a minimal set of features that even the most limited devices can be expected to support. One large advantage XHTML Basic has over WML and C-HTML is that XHTML Basic pages can be rendered differently in web browsers and on handhelds, without the need for two different versions of the same page. It will replace WML and C-HTML as more compliant user agents are developed.

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.

In 2006, the specification was revised to version 1.1. Six new features have been incorporated into the language in order to better serve the small-device community. "XHTML Basic 1.1" became a W3C Recommendation in July 2008, superseding XHTML-MP 1.2.cite web
url=http://www.w3.org/TR/2008/REC-xhtml-basic-20080729
title=XHTML™ Basic 1.1, W3C Recommendation 29 July 2008
publisher=World Wide Web Consortium
date=2008-07-29
accessdate=2008-07-29] XHTML Basic 1.1 is almost but not quite a subset XHTML 1.1, defined using XHTML Modularization including a reduced set of modules for document structure, images, forms, basic tables, and object support. The most notable addition over XHTML 1.1 is the inputmode 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 a new language based on XHTML 1.1. [ [http://lists.w3.org/Archives/Public/public-xhtml2/2008Jul/0010.html [XHTML Agenda: 2008-07-09 from Roland Merrick on 2008-07-09 ([e-mail] from July 2008) ] ] If XHTML 1.2 is created, it will include WAI-ARIA and role attributes to better support accessible web applications, and improved Semantic Web support through RDFa. The inputmode attribute from XHTML Basic 1.1, along with the target attribute (for specifying frame targets) may also be present.

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.See both " [http://lists.w3.org/Archives/Public/www-html/2003Jan/0123.html XHTML 2.0 Considered Harmful] " and " [http://tantek.com/log/2003/05.html#L20030508t1620 XHTML 2.0 Considered Hopeful] " by browser developer Tantek Çelik, who criticises early drafts of XHTML 2.0 for the absence of the style attribute and the cite element. Developer Daniel Glazman offers [http://lists.w3.org/Archives/Public/www-html/2002Dec/0113.html similar criticism] , but also shows support for some backward-incompatible changes such as the decision to remove the ins and del elements.] Some parts of the language (such as the role 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 is expected to appear in 2008.

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 nl 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.,

Articles

, 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 src attribute, e.g.,

London Bridge

is the same as .
* The alt attribute of the img element has been removed: alternative text will be given in the content of the img element, much like the object element, e.g., HMS Audacious.
* A single heading element (h) 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 i, b and tt, still allowed in XHTML 1.x (even Strict), will be absent from XHTML 2.0. The only somewhat presentational elements remaining will be sup and sub 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 property and about attributes to facilitate the conversion from XHTML to RDF/XML.

HTML 5—Vocabulary and APIs for HTML and XHTML

HTML 5 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.” [ [http://www.whatwg.org/news/start WHAT open mailing list announcement] ] 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 forum sites or online shops.cite web
url=http://www.w3.org/TR/2008/WD-html5-20080122/#relationship0
title=HTML 5, 1.1.2. Relationship to XHTML2
author=Ian Hickson
date=2008-01-22
publisher=World Wide Web Consortium
accessdate= 2008-07-19
quote=… XHTML2… defines a new HTML vocabulary with better features for hyperlinks, multimedia content, annotating document edits, rich metadata, declarative interactive forms, and describing the semantics of human literary works such as poems and scientific papers… However, it lacks elements to express the semantics of many of the non-document types of content often seen on the Web. For instance, forum sites, auction sites, search engines, online shops, and the like, do not fit the document metaphor well, and are not covered by XHTML2… "This" specification aims to extend HTML so that it is also suitable in these contexts…]

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. [ [http://lists.w3.org/Archives/Public/public-html/2007Apr/0429.html Proposal to Adopt HTML5] ] The group resolved to do this the following month, [ [http://lists.w3.org/Archives/Public/public-html/2007May/0909.html results of HTML 5 text, editor, name questions] ] 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.

HTML 5 has both a regular text/html serialization and an XML serialization, which is known as XHTML 5. 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 can be considered 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 section and aside. (The XHTML 1.2 equivalent of these structural elements would be

and

.)

The specification is expected to add WAI-ARIA support in a future draft. There is currently no indication as to whether HTML 5 will support RDFa, or be limited just to microformats.

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 such as Dreamweaver provide code validation based on the W3C standards.

Root element

The root element of an XHTML document must be html, and must contain an xmlns attribute to associate it with the XHTML namespace. The namespace URI for XHTML is http://www.w3.org/1999/xhtml. For XHTML 1.1 and later there should also ideally be a version attribute to clearly identify the version of XHTML being used. The example tag below additionally features an xml:lang attribute to identify the document with a natural language:

For XHTML 1.1 and 2.0 an optional schemaLocation attribute can be added, to associate the namespace with an XML Schema. The example below is for XHTML 2.0:

This example also demonstrates the use of multiple namespaces within a document. The xmlns:xsi declaration indicates that any elements or attributes prefixed with xsi: fall within the XML Schema instance namespace rather than the XHTML namespace. This mechanism of prefixes allows elements and attributes from different XML vocabularies to be mixed together in the same document, without the potential for naming clashes.

DOCTYPEs

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

These are the most common XHTML Document Type Declarations:

;XHTML 1.0 Strict:<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">;XHTML 1.0 Transitional:<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">;XHTML 1.0 Frameset:<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">;XHTML 1.1:<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">;XHTML Basic 1.1:<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.1//EN" "http://www.w3.org/TR/xhtml-basic/xhtml-basic11.dtd">;XHTML 2.0XHTML 2.0, As of April 2008, is in a draft phase. If an XHTML 2.0 Recommendation is published with the same document type declaration as in the current Working Draft, the declaration will appear as::<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 2.0//EN" "http://www.w3.org/MarkUp/DTD/xhtml2.dtd">;HTML 5HTML 5 has no formal Document Type Definition, but "must" use <!DOCTYPE html> when served as text/html. (This is case insensitive, and not required for content served as application/xhtml+xml.)

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 application/xhtml+xml 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 text/html.

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 are acceptable in XHTML: <br></br>, <br/> and <br />. Older HTML-only browsers interpreting it as HTML will generally accept <br> and <br />.
* Not closing non-empty elements
** Incorrect:

This is a paragraph.

This is another paragraph.
** Correct:

This is a paragraph.

This is another paragraph.

* Improperly nesting elements (Note that this would also be invalid in HTML)
** Incorrect: This is some text.
** Correct: This is some text.
* Not putting quotation marks around attribute values
** Incorrect:
** Correct:
** Correct:
* Using the ampersand character outside of entities (Note that this would also be invalid in HTML)
** Incorrect:
** Correct:
** Incorrect: News
** Correct: News
* Failing to recognize that XHTML elements and attributes are case sensitive
** Incorrect:

The Best Page Ever

** Correct:

The Best Page Ever

* Using attribute minimization
** Incorrect: READ-ONLY
** Correct: READ-ONLY
* Mis-using 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 head element.

::Note: The format <script …></script>, rather than the more concise <script … />, is required for HTML compatibility when served as MIME type text/html.:* 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 application/xhtml+xml and target only fully conformant browsers, or serve the page as text/html 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 role attribute, RDFa and WAI-ARIA degrade gracefully in a similar manner.

HTML 5 and XHTML 2 are 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 “document.createElement()” to register a new HTML element within Internet Explorer, or complete JavaScript frameworks, such as the FormFaces implementation of XForms.)

Examples

The followings are examples of XHTML 1.0 Strict. Both of them have the same visual output. The former one follows the [http://www.w3.org/TR/xhtml1/#guidelines HTML Compatibility Guidelines] in Appendix C of the XHTML 1.0 Specification while the latter one breaks backward compatibility but provides cleaner markup.

Example 1.

This is an example of an XHTML 1.0 Strict document.

Example 2.

This is an example of an XHTML 1.0 Strict document.