XHTML

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 prior to HTML 5 was defined as 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".The W3C also continues to maintain the HTML 4.01 Recommendation and the specifications for HTML 5 and XHTML 5 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
In the late 1990s, many considered that the future of HTML lay in the creation of a version adhering to the syntax rules of XML. The then current version of HTML, 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 with it. By shifting the underlying base from SGML to the simpler XML, 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.
Another key advantage was extensibility. By utilising namespaces, XHTML documents could include 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
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. 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 that 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 attributes can omit quotes in certain cases. 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., and
. Attribute minimization (e. g., ) is also prohibited, as the attribute selected contains no explicit value; instead this would be written as .
HTML elements that 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 element, and a table will not have a element unless the author specifies one.
The XHTML 1.0 recommendation devotes a section to differences between HTML and XHTML. The WHATWG wiki similarly considers differences that arise with the use of (X)HTML5.
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.
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 (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 support for all of the possible XHTML media types. 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 Internet Explorer 8 Release Candidate 1 (released in January 2009) 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-compatible, 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.
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 IE6. 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.