XHTML
[8] 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.
[15] XHTML was developed to make HTML more extensible and increase interoperability with other data formats.
[18] The XML standard, approved in 1998, provided a simpler data format closer in simplicity to HTML 4.
[21] By using namespaces, XHTML documents could provide extensibility by including fragments from other XML-based languages such as Scalable Vector Graphics and MathML.
[22] Finally, the renewed work would provide an opportunity to divide HTML into reusable components (XHTML Modularization) and clean up untidy parts of the language.
Because the expressible contents of the DOM in syntax are slightly different, there are some changes in actual behavior between the two models.
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.
[36] As long as support is not widespread, most web developers avoid using XHTML that is not HTML-compatible,[37] so advantages of XML such as namespaces, faster parsing, and smaller-footprint browsers do not benefit the user.
"[46] 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."
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.
The feature is intended to help XHTML extend its reach onto emerging platforms, such as mobile devices and Web-enabled televisions.
[54] With limited browser support for the alternate application/xhtml+xml media type, XHTML 1.1 proved unable to gain widespread use.
[55] 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.
[56] (It was first released briefly on 7 May 2009 as a "Proposed Edited Recommendation"[57] before being rescinded on 19 May due to unresolved issues.)
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.
[62] 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.
[64] 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.
This lack of compatibility with XHTML 1.x and HTML 4 caused some early controversy in the web developer community.
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.
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.
[70] 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.
In practice, many web development programs provide code validation based on the W3C standards.
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.
Internet Explorer prior to version 7 enters quirks mode, if it encounters an XML declaration in a document served as text/html.
Later XHTML 1.x modules such as those for the role attribute, RDFa, and WAI-ARIA degrade gracefully in a similar manner.
(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.)
Notes: HTML5 and XHTML5 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[clarify], 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 ?>).
