XHTML#common errors

XHTML 1.0 was "a reformulation of the three HTML 4 document types as applications of XML 1.0".{{cite web|access-date=2007-06-16|date=2000-01-26|publisher=World Wide Web Consortium|title=XHTML 1.0 Specification, Section 1: What is XHTML?|url=http://www.w3.org/TR/xhtml1/#xhtml}} The World Wide Web Consortium (W3C) also simultaneously maintained the HTML 4.01 Recommendation. In the XHTML 1.0 Recommendation document, as published and revised in 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 2005, the Web Hypertext Application Technology Working Group (WHATWG) formed, independently of the W3C, to work on advancing ordinary HTML not based on XHTML. The WHATWG eventually began working on a standard that supported both XML and non-XML serializations, HTML5, in parallel to W3C standards such as XHTML 2.0. In 2007, the W3C's HTML working group voted to officially recognize HTML5 and work on it as the next-generation HTML standard.{{cite web|title=results of HTML 5 text, editor, name questions|url=http://lists.w3.org/Archives/Public/public-html/2007May/0909.html|work=W3C }} In 2009, the W3C allowed the XHTML 2.0 Working Group's charter to expire, acknowledging that HTML5 would be the sole next-generation HTML standard, including both XML and non-XML serializations. Of the two serializations, the W3C suggests that most authors use the HTML syntax, rather than the XHTML syntax.{{cite web|url=http://www.w3.org/TR/html5/introduction.html#html-vs-xhtml|access-date=2011-02-16|date=2011-01-13|publisher=World Wide Web Consortium|title=HTML5 Working Draft, Section 1.6: HTML vs XHTML}}

The W3C recommendations of both XHTML 1.0 and XHTML 1.1 were retired on 27 March 2018,{{cite web |title=XHTML™ 1.0 The Extensible HyperText Markup Language (Second Edition) Publication History|date=27 March 2018 |publisher=World Wide Web Consortium|url=https://www.w3.org/standards/history/xhtml1}}{{cite web |title=XHTML™ 1.1 - Module-based XHTML - Second Edition Publication History|date=27 March 2018 |publisher=World Wide Web Consortium|url=https://www.w3.org/standards/history/xhtml11}} along with HTML 4.0,{{cite web |title=HTML 4.0 Publication History|date=27 March 2018 |publisher=World Wide Web Consortium|url=https://www.w3.org/standards/history/html40}} HTML 4.01,{{cite web |title=HTML 4.01 Publication History|date=27 March 2018 |publisher=World Wide Web Consortium|url=https://www.w3.org/standards/history/html401}} and HTML5.{{cite web |title=HTML5 Publication History|date=27 March 2018 |publisher=World Wide Web Consortium|url=https://www.w3.org/standards/history/html5}}

XHTML was developed to make HTML more extensible and increase interoperability with other data formats.{{cite web|access-date=2007-06-16|date=2000-01-26|publisher=World Wide Web Consortium|title=XHTML 1.0 Specification, Section 1.1: Why the need for XHTML?|url=http://www.w3.org/TR/xhtml1/#why}} In addition, browsers were forgiving of errors in HTML, and most websites were displayed despite technical errors in the markup; XHTML introduced stricter error handling.{{Cite web

| url = http://diveintohtml5.info/past.html

| title = How Did We Get Here? - Dive Into HTML5

| last = Pilgrim

| first = Mark

| website = diveintohtml5.info

| access-date = 2016-06-11

}} 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.{{cite web|access-date=2008-12-29|author=Arjun Ray|date=1999-10-06|quote=... However, since ISO 8879 does not afford applications the leeway to prohibit internal subsets, it follows that the letter of the HTML [4] spec automatically disentitles it to be a conforming SGML application...|title=Dropping the Normative Reference to SGML (was: I-D ACTION.)|url=http://markmail.org/message/drvncr3f6yscveeg|archive-date=2021-02-25|archive-url=https://web.archive.org/web/20210225124458/https://markmail.org/message/drvncr3f6yscveeg|url-status=dead}} The XML standard, approved in 1998, provided a simpler data format closer in simplicity to HTML 4.{{cite web

|url = http://www.dev-archive.net/articles/xhtml.html

|title = XHTML—Myths and Reality

|author = Tina Holmboe

|publisher = The Developer's Archive

|date = 2008-10-06

|access-date = 2008-12-29

|quote = ... Since the design goals of XML itself partially mirrored those of the original HTML, it was logical for work to begin on formulating an XML–based markup language...

|archive-date = 2017-01-12

|archive-url = https://web.archive.org/web/20170112181538/http://www.dev-archive.net/articles/xhtml.html

|url-status = dead

}} By shifting to an XML format, it was hoped HTML would become compatible with common XML tools;{{cite web

| url = http://www.xml.com/pub/a/2000/01/10/perlwebtools.html

| title = Creating Web Utilities Using XML::XPath

| author = Kip Hampton

| publisher = XML.com

| date = 2001-01-10

| access-date = 2008-12-29

| quote = ... The problem: You want to take advantage of the power and simplicity that XML tools can offer, but you face a site full of aging HTML documents. The solution: Convert your documents to XHTML and put Perl and XML::XPath to work...

}} servers and proxies would be able to transform content, as necessary, for constrained devices such as mobile phones.{{cite web

| url = http://www.xml.com/pub/a/2004/04/14/mobile.html

| title = Developing Wireless Content using XHTML Mobile

| author = Jean-Luc David

| publisher = XML.com

| date = 2004-04-14

| access-date = 2008-12-29

| quote = ... A useful feature of XHTML is that it can be manipulated as XML. Extensible Stylesheet Language Templates can be used to transform XHTML into WML or any other proprietary mobile formats...

}}

By using namespaces, XHTML documents could provide extensibility by including fragments from other XML-based languages such as Scalable Vector Graphics and MathML.{{cite web

|url = https://developer.mozilla.org/En/SVG:Namespaces_Crash_Course

|title = Namespaces Crash Course

|publisher = Mozilla Developer Center

|access-date = 2008-12-29

|quote = ... It has been a long-standing goal of the W3C to make it possible for different types of XML-based content to be mixed together in the same XML file. For example, SVG and MathML might be incorporated directly into an XHTML-based scientific document...

|archive-date = 2008-10-02

|archive-url = https://web.archive.org/web/20081002054459/http://developer.mozilla.org/En/SVG:Namespaces_Crash_Course

|url-status = dead

}} Finally, the renewed work would provide an opportunity to divide HTML into reusable components (XHTML Modularization) and clean up untidy parts of the language.{{cite web

| url = http://www.w3.org/MarkUp/2004/xhtml-faq

| title = HTML and XHTML Frequently Answered Questions

| author = Steven Pemberton

| publisher = World Wide Web Consortium

| date = 2004-07-21

| access-date = 2008-12-29

| quote = ... with an XML-based HTML other XML languages could include bits of XHTML, and XHTML documents could include bits of other markup languages. We could also take advantage of the redesign to clean up some of the more untidy parts of HTML and add some new needed functionality, like better forms...

}}

===Relationship to HTML===

There are various differences between XHTML and HTML. The Document Object Model (DOM) is a tree structure that represents the page internally in applications, and XHTML and HTML are two different ways of representing that in markup. 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 more expressive than HTML (for example, arbitrary namespaces are not allowed in HTML). XHTML uses an XML syntax, while HTML uses a pseudo-SGML syntax (officially SGML for HTML 4 and under, but never in practice, and standardized away from SGML in HTML5). Because the expressible contents of the DOM in syntax are slightly different, there are some changes in actual behavior between the two models. Syntax differences, however, can be overcome by implementing an alternate translational framework within the markup.

First, there are some differences in syntax:{{cite web

| url = http://www.w3.org/TR/NOTE-sgml-xml-971215

| first = James

| last = Clark

| title = Comparison of SGML and XML

| publisher = World Wide Web Consortium Note

| date = 1997-12-15

}}

• {{anchor|self-closing tag|self-closing syntax|self-closing}}Broadly, the XML rules require that every element be closed, either with a separate closing tag (e.g. {{Code||xml}}) or by using the self-closing syntax (e.g. {{Code|
  |xml}}), while HTML syntax permits some elements to be unclosed because either they are always empty (e.g. {{Code||html}}) or their end can be determined implicitly ("omissibility", e.g. {{Code|

  |html}}).

• 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. {{Code||html}} or {{Code||html}}, while in XML this must be expressed as {{Code||xml}}); (2) element minimization may be used to remove elements entirely (such as {{Code||xml}} inferred in a table if not given); and (3) the rarely used SGML syntax for element minimization ("shorttag"), which most browsers do not implement.{{cite web

| title = Shorthand markup

| url = http://www.w3.org/TR/REC-html40/appendix/notes.html#h-B.3.7

| work = HTML 4, Appendix B: Performance, Implementation, and Design Notes

| publisher = W3C

| access-date = 30 September 2011

}}

• 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 ([http://www.whatwg.org/specs/web-apps/current-work/multipage/ [HTML5]]) for full details, or the working summary ([http://wiki.whatwg.org/wiki/HTML_vs._XHTML HTML vs. XHTML]).

In addition to the syntactical differences, there are some behavioral differences, mostly arising from the underlying differences in serialization. For example:

• Behavior on parse errors differs. A fatal parse error in XML (such as an incorrect tag structure) causes document processing to be aborted.
• Most content requiring namespaces will not work in HTML, except the built-in support for SVG and MathML in the HTML5 parser along with certain magic prefixes such as xlink.
• JavaScript processing is different in XHTML, with minor changes in case sensitivity to some functions, and further precautions to restrict processing to well-formed content. Scripts must not use the {{code|lang=javascript|code=document.write()}} method; it is not available for XHTML. The innerHTML property is available, but will not insert non-well-formed content. On the other hand, it can be used to insert well-formed namespaced content into XHTML.
• Cascading Style Sheets (CSS) are also applied differently. Due to XHTML's case-sensitivity, all CSS selectors become case-sensitive for XHTML documents.{{cite web

| title = Case Sensitivity

| url = http://reference.sitepoint.com/css/casesensitivity

| publisher = SitePoint Pty. Ltd

| access-date = 30 September 2011

}} Some CSS properties, such as backgrounds, set on the {{Code||html}} element in HTML are 'inherited upwards' into the {{Code||html}} element; this appears{{clarify|It either is or it isn't the case|date=December 2018}} not to be the case for XHTML.{{cite web

| url = http://www.nicholaswilson.me.uk/2010/05/css-differences-between-xhtml-and-html

| title = CSS differences between XHTML and HTML

| last = Wilson

| first = Nicholas

| date = 29 May 2010

}}

The similarities between HTML 4.01 and XHTML 1.0 led many websites 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

| access-date = 2007-06-16

}}{{cite web

| url = http://www.w3.org/TR/xhtml-media-types

| title = XHTML Media Types, W3C Working Group Note

| publisher = World Wide Web Consortium

| date = 2002-08-01

| access-date = 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 are being compared.{{Cite web|url=https://support.google.com/webmasters/answer/79812?hl=en|title = Meta and Inline Tags that Google Understands {{pipe}} Google Search Central}}Greta de Groat (2002). "Perspectives on the Web and Google: Monika Henzinger, Director of Research, Google", Journal of Internet Cataloging, Vol. 5(1), pp. 17-28, 2002.

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 [https://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.{{cite web

| url = http://www.w3.org/People/mimasa/test/xhtml/media-types/results

| title = XHTML media type test - results

| work = w3.org

}} The notable exception is Internet Explorer versions 8 and earlier 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 Internet Explorer 8 (released in March 2009) exhibit this behavior.{{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

| access-date = 2007-06-16

| quote = I've also been reading comments for some time in the IEBlog asking for support for the "application/xml+xhtml" MIME type in IE. I should say that IE7 will not add support for this MIME type – we will, of course, continue to read XHTML when served as "text/html", presuming it follows the HTML compatibility recommendations.

}} Microsoft developer Chris Wilson explained in 2005 that IE7's priorities were improved browser 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

| access-date = 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...

}} however, Microsoft added support for true XHTML in IE9.{{cite web

| url = http://blogs.msdn.com/ie/archive/2010/03/16/html5-hardware-accelerated-first-ie9-platform-preview-available-for-developers.aspx

| title = HTML5, Hardware Accelerated: First IE9 Platform Preview Available for Developers

| date = 2019-03-16

| publisher = Microsoft

| first = Dean

| last = Hachamovitch

| work = IEBlog on Microsoft Developer Network

| access-date = 2010-03-22

| quote = ...At this time, we're looking for developer feedback on our implementation of HTML5's parsing rules, Selection APIs, XHTML support, and inline SVG. Within CSS3, we're looking for developer feedback on IE9's support for Selectors, Namespaces, Colors, Values, Backgrounds and Borders, and Fonts....

}}

As long as support is not widespread, most web developers avoid using XHTML that is not HTML-compatible,{{cite web

| url = http://www.goer.org/Markup/TheXPhiles/

| title = List of XHTML Sites (the X-Philes)

| access-date = 2008-08-26

}} so advantages of XML such as namespaces, faster parsing, and smaller-footprint browsers do not benefit the user.[http://sangers.nu/blog/tech/20080909-the-meta-tag-attribute--keywords "In 2007, 37 leaders in search engine optimisation concluded that having keywords in the keywords attribute is little to none." Sanger. nu blog, September 9, 2008, Retrieved August 2 2011] {{webarchive |url=https://web.archive.org/web/20090221001513/http://sangers.nu/blog/tech/20080909-the-meta-tag-attribute--keywords |date=February 21, 2009 }}{{cite web

| url = http://nettrafficacademy.com/blog/meta-for-seo/

| title = Meta used for SEO

| date = 18 December 2015

| access-date = March 18, 2016

| archive-url = https://web.archive.org/web/20160331224917/http://nettrafficacademy.com/blog/meta-for-seo/

| archive-date = March 31, 2016

| url-status = live

}}Danny Sullivan, [http://searchenginewatch.com/showPage.html?page=2167931 How To Use HTML Meta Tags] {{Webarchive|url=https://web.archive.org/web/20080913174704/http://searchenginewatch.com/showPage.html?page=2167931 |date=2008-09-13 }}, Search Engine Watch, December 5, 2002

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

| access-date = 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

| access-date = 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

| access-date = 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 Internet Explorer 6.{{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

| access-date = 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

| access-date = 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 widespread, 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 a new HTML specification, posted in his blog that "[t]he 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

| access-date = 2007-06-16

| quote = Some things are clearer with the 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 continue a transition to [a] well-formed world, and develop 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.

| url-status = dead

| archive-url = https://web.archive.org/web/20070609092703/http://dig.csail.mit.edu/breadcrumbs/node/166

| archive-date = 2007-06-09

}} The current HTML5 working draft says "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, editor of the HTML5 specification criticizing the improper use of XHTML in 2002, is a member of the group developing this specification and is listed as one of the co-editors of the current working draft.{{cite web

| url = http://www.w3.org/TR/html5/

| title = HTML5: A vocabulary and associated APIs for HTML and XHTML

| author1 = Ian Hickson

| author2 = David Hyatt

| date = 2011-01-13

| access-date = 2011-02-16

}}

Simon Pieters researched the XML-compliance of mobile browsers{{cite web

| url = http://simon.html5.org/articles/mobile-results

| title = Results of mobile tests

| author = Simon Pieters

| access-date = 2009-10-31

}} and concluded "the claim that XHTML would be needed for mobile devices is simply a myth".

File: Wikipedia main page XHTML source.png, Wikipedia began using on [https://web.archive.org/web/20120918145247/http://en.wikipedia.org/wiki/Main_Page 18 September 2012]; before that, was used, starting from [https://web.archive.org/web/20040605151204/http://en.wikipedia.org/wiki/Main_Page June of 2004]. Wikipedia used the XHTML 1.0 Transitional doctype and syntax, though the content was not served as application/xhtml+xml.]]

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 name of the specification had changed 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

| access-date = 2008-07-19

}} There are three formal Document Type Definitions (DTD) 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. {{As of|2015|11}}, XHTML 1.0 Strict is the document type used for the homepage of the website of the World Wide Web Consortium.
• XHTML 1.0 Transitional is the XML 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 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.{{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

| access-date = 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 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

| access-date = 2008-07-19

}}

The first modular XHTML variants were XHTML 1.1 and XHTML Basic 1.0.

In October 2008 Modularization of XHTML was superseded by XHTML Modularization 1.1, which adds an XML Schema implementation. It was superseded by a second edition in July 2010.{{cite web

| url = http://www.w3.org/TR/2010/REC-xhtml-modularization-20100729

| title = XHTML Modularization 1.1 - Second Edition, W3C Recommendation 29 July 2010

| publisher = World Wide Web Consortium

| date = 2010-07-29

| access-date = 2010-12-31

}}

XHTML 1.1 evolved out of the work surrounding the initial Modularization of XHTML specification. The W3C released the first draft in September 1999; the 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

| access-date = 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 the removal of the name attribute from the a and map elements, and (in the first edition of the language) the removal of the lang attribute in favor of xml:lang.

Although XHTML 1.1 is largely compatible with XHTML 1.0 and HTML 4, in August 2002 the Working Group 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 Working Group Note 1 August 2002

| publisher = World Wide Web Consortium

| date = 2002-08-01

| access-date = 2008-07-19

}} With limited browser support for the alternate application/xhtml+xml 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) was issued, relaxing this restriction and allowing XHTML 1.1 to be served as text/html.{{cite web

| url = http://www.w3.org/TR/2009/NOTE-xhtml-media-types-20090116

| title = XHTML Media Types – Second Edition, W3C Working Group Note 16 January 2009

| publisher = World Wide Web Consortium

| date = 2009-01-16

| access-date = 2009-01-28

}}

This document supersedes the HTML Compatibility Guidelines originally found in XHTML 1.0 Appendix C.

The second edition of XHTML 1.1 was issued on 23 November 2010, which addresses various errata and adds an XML Schema implementation not included in the original specification.{{cite web

| title = XHTML 1.1, XHTML Basic 1.1, XHTML Print Recommendations Revised

| url = http://www.w3.org/News/2010.html#entry-8962

| work = W3C NEWS ARCHIVE: 2010

| publisher = World Wide Web Consortium

| access-date = 12 December 2010

}} (It was first released briefly on 7 May 2009 as a "Proposed Edited Recommendation"{{cite web

|url = http://www.w3.org/TR/2009/PER-xhtml11-20090507/

|title = XHTML 1.1 - Module-based XHTML – Second Edition

|publisher = World Wide Web Consortium

|date = 2009-05-07

|access-date = 2009-05-25

|archive-date = 2009-05-12

|archive-url = https://web.archive.org/web/20090512014151/http://www.w3.org/TR/2009/PER-xhtml11-20090507/

|url-status = dead

}} before being rescinded on 19 May due to unresolved issues.)

{{Main|XHTML Basic}}

Since information appliances may lack the system resources to implement all XHTML abstract modules, the W3C defined a feature-limited XHTML specification called XHTML Basic. It provides a minimal feature subset sufficient for the most common content-authoring. The specification became a W3C recommendation 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

| access-date = 2008-07-19

}}

Of all the versions of XHTML, XHTML Basic 1.0 provides the fewest features. With XHTML 1.1, it is one of the two first implementations of modular XHTML. In addition to the Core Modules (Structure, Text, Hypertext, and List), it implements the following abstract modules: Base, Basic Forms, Basic Tables, Image, Link, Metainformation, Object, Style Sheet, and Target.{{cite web

| url = http://www.w3.org/2007/09/dtd-comparison.html

| title = XHTML Flavors comparisons

| publisher = World Wide Web Consortium

| date = 2007-01-09

| access-date = 2013-01-30

}}[http://www.w3.org/TR/2000/REC-xhtml-basic-20001219/ XHTML Basic]. W3.org. Retrieved on 2013-07-17.

XHTML Basic 1.1 replaces the Basic Forms Module with the Forms Module and adds the Intrinsic Events, Presentation, and Scripting modules. It also supports additional tags and attributes from other modules. This version became a W3C recommendation on 29 July 2008.[http://www.w3.org/TR/2008/REC-xhtml-basic-20080729/ XHTML Basic 1.1]. W3.org. Retrieved on 2013-07-17.

The current version of XHTML Basic is 1.1 Second Edition (23 November 2010), in which the language is re-implemented in the W3C's XML Schema language. This version also supports the lang attribute.{{cite web

| url = http://www.w3.org/TR/2010/REC-xhtml-basic-20101123

| title = XHTML Basic 1.1 - Second Edition

| work = w3.org

}}

XHTML-Print, which became a W3C Recommendation in September 2006, is a specialized version of XHTML Basic designed for documents printed from information appliances to low-end 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

| access-date = 2008-07-19

}}

{{Main|XHTML Mobile Profile}}

XHTML Mobile Profile (abbreviated XHTML MP or XHTML-MP) is a third-party variant of the W3C's XHTML Basic specification. Like XHTML Basic, XHTML was developed for information appliances with limited system resources.

In October 2001, a limited company called the Wireless Application Protocol Forum began adapting XHTML Basic for WAP 2.0, the second major version of the Wireless Application Protocol. WAP Forum based their DTD on the W3C's Modularization of XHTML, incorporating the same modules the W3C used in XHTML Basic 1.0—except for the Target Module. Starting with this foundation, the WAP Forum replaced the Basic Forms Module with a partial implementation of the Forms Module, added partial support for the Legacy and Presentation modules, and added full support for the Style Attribute Module.

In 2002, the WAP Forum has subsumed into the Open Mobile Alliance (OMA), which continued to develop XHTML Mobile Profile as a component of their OMA Browsing Specification.

To this version, finalized in 2004, the OMA added partial support for the Scripting Module and partial support for Intrinsic Events. XHTML MP 1.1 is part of v2.1 of the OMA Browsing Specification (1 November 2002).{{cite web

| url = http://www.openmobilealliance.org/Technical/release_program/browsing_archive.aspx

| title = OMA Browsing Archive

| date = 26 September 2011

| work = OMA Releases

| publisher = Open Mobile Alliance Ltd.

}}

This version, finalized on 27 February 2007, expands the capabilities of XHTML MP 1.1 with full support for the Forms Module and OMA Text Input Modes. XHTML MP 1.2 is part of v2.3 of the OMA Browsing Specification (13 March 2007).

XHTML MP 1.3 (finalized on 23 September 2008) uses the XHTML Basic 1.1 document type definition, which includes the Target Module. Events in this version of the specification are updated to DOM Level 3 specifications (i.e., they are platform- and language-neutral).

The XHTML 2 Working Group considered the creation of a new language based on XHTML 1.1.{{cite web

| url = http://lists.w3.org/Archives/Public/public-xhtml2/2008Jul/0010.html

| title = [XHTML] Agenda: 2008-07-09

| work = w3.org

}} If XHTML 1.2 was created, it would 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) might also be present. The XHTML2 WG had not been chartered to carry out the development of XHTML1.2. Since the W3C announced that it does not intend to recharter the XHTML2 WG,{{cite web

| url = http://www.w3.org/2009/06/xhtml-faq.html

| title = Frequently Asked Questions (FAQ) about the future of XHTML

| work = w3.org

}} and closed the WG in December 2010, this means that XHTML 1.2 proposal would not eventuate.

Between August 2002 and July 2006, the W3C released 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 criticizes 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. The ninth draft of XHTML 2.0 was expected to appear in 2009, but on 2 July 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. Instead, XHTML 2.0 and its related documents were released as W3C Notes in 2010.{{cite web

| url = http://www.w3.org/TR/2010/NOTE-xhtml2-20101216

| title = XHTML 2.0, W3C Working Group Note 16 December 2010

| publisher = World Wide Web Consortium

| date = 2010-12-16

| access-date = 2010-12-31

}}{{cite web

| url = http://www.w3.org/News/2010#entry-8982

| title = XHTML2 Working Group Documents Published as W3C Notes

| publisher = World Wide Web Consortium

| date = 2010-12-16

| access-date = 2010-12-31

}}

New features to have been introduced by XHTML 2.0 included:

• HTML forms were to be replaced by XForms, an XML-based user input specification allowing forms to be displayed appropriately for different rendering devices.
• HTML frames were to be replaced by XFrames.
• The DOM Events were to be replaced by XML Events, which uses the XML Document Object Model.
• A new list element type, the nl element type, was to be included to specifically designate a list as a navigation list. This would have been 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 was to be able to act as a hyperlink, e. g., {{Code|code=
• Articles
|xml}}, similar to XLink. However, XLink itself is not compatible with XHTML due to design differences.
• Any element was to be able to reference alternative media with the src attribute, e. g., {{Code|code=

  London Bridge

  |xml}} is the same as {{Code|code=|xml}}.
• The alt attribute of the img element was removed: alternative text was to be given in the content of the img element, much like the object element, e. g., {{Code|code=HMS Audacious|xml}}.
• A single heading element (h) was added. The level of these headings was determined by the depth of the nesting. This would have allowed 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), were to be absent from XHTML 2.0. The only somewhat presentational elements remaining were to 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 were meant to be semantic instead (e. g. strong for strong emphasis) while allowing the user agent to control the presentation of elements via CSS (e.g. rendered as boldface text in most visual browsers, but possibly rendered with changes of tone in a text-to-speech reader, larger + italic font per rules in a user-end stylesheet, etc.).
• The addition of RDF triple with the property and about attributes to facilitate the conversion from XHTML to RDF/XML.

{{Main|XHTML5}}

HTML5 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 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.{{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

| access-date = 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...

}}

HTML5 has both a regular text/html serialization and an XML serialization, which is also known as XHTML5.{{cite web

| url = http://www.w3.org/TR/html5/the-xhtml-syntax.html#xhtml

| title = 9 The XHTML syntax — HTML5

| work = w3.org

}} 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 section and aside tags.

The XHTML5 language, like HTML5, uses a DOCTYPE declaration without a DTD. Furthermore, the specification deprecates earlier XHTML DTDs by asking the browsers to replace them with one containing only entity definitions for named characters during parsing.

XHTML+RDFa is an extended version of the XHTML markup language for supporting RDF through a collection of attributes and processing rules in the form of well-formed XML documents. This host language is one of the techniques used to develop Semantic Web content by embedding rich semantic markup.

{{unreferenced section|date=July 2013}}

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 (for XHTML5, the Validator. nu Living Validator should be used instead). In practice, many web development programs provide code validation based on the W3C standards.

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. The example tag below additionally features an xml:lang attribute to identify the document with a natural language:

{{Main|DOCTYPE}}

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.

{{See also|Processing Instruction}}

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:

:{{Code|1=|2=xml}}

The declaration may be optionally omitted because it declares 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.

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.

XHTML 2.0 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 document.createElement() to register a new HTML element within Internet Explorer, or complete JavaScript frameworks, such as the FormFaces implementation of XForms.)

The following are examples of XHTML 1.0 Strict, 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.

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

This is an example of an

XHTML 1.0 Strict document.

src="http://www.w3.org/Icons/valid-xhtml10"

alt="Valid XHTML 1.0 Strict"/>

Example 2.

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

This is an example of an

XHTML 1.0 Strict document.

src="http://www.w3.org/Icons/valid-xhtml10"

alt="Valid XHTML 1.0 Strict"/>

Notes:

1. The "loadpdf" function is actually a workaround for Internet Explorer. It can be replaced by adding within