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's 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.". The WHATWG wiki similarly considers differences that arise with the use of (X)HTML5..
Because XHTML and HTML are closely related technologies, sometimes they're 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
Adoption of XHTML continues at an uneven pace.
The similarities between HTML 4.01 and XHTML 1.0 led many web authors, content management systems and web sites to adopt the initial W3C XHTML 1.0 recommendations. To aid authors in the transition, the W3C included an appendix to the XHTML 1.0 recommendations describing how to publish XHTML 1.0 documents as HTML-compatible documents and serve them to HTML browsers that were not designed for XHTML.
Browser support for XHTML 1.0 remains incomplete even though it has been many years since the Recommendation status of XHTML 1.0. For example, Internet Explorer by Microsoft (MSIE) has had XML parsing capabilities since version 5.0 in 1999, but even in mid-2007, the current version (IE7) still doesn't support XHTML documents served as XML, nor does IE 8 beta 1; it only renders them correctly when they're served as HTML and are authored in accordance with the HTML compatibility guidelines. MSIE 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.
Most other browsers now have mature support for all of the possible XHTML MIME types . Early implementations (such as Mozilla 0.7 and Opera 6.0, both released in 2001) don't incrementally render XHTML as it's received over the network, giving a degraded user experience. Later browsers such as Opera 9.0, Safari 3.0 and Firefox 3.0 (presently still in beta) have gained incremental rendering to resolve the issue.
Obstacles from browser vendors have slowed the effective rate of the adoption. Without broader browser support, XHTML documents are commonly served as HTML, so advantages of XML such as namespaces, faster parsing and smaller-footprint browsers don't benefit the user.
Recently, notable developers have begun to question why Web authors ever made the leap into authoring in XHTML. In October 2006, HTML inventor and W3C chair Tim Berners-Lee, explaining the motivation for the resumption of HTML (not XHTML) development, posted in his blog: "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 didn't move, largely because the browsers didn't complain."
Versions of XHTML
XHTML 1.0
The original XHTML W3C Recommendation, XHTML 1.0, was simply a reformulation of HTML 4.01 in XML. There are three different "DTDs" of XHTML 1.0, each equal in scope to their respective HTML 4.01 versions.
XHTML 1.0 Strict is the same as HTML 4.01 Strict, but it follows XML syntax rules.
XHTML 1.0 Transitional is the same as HTML 4.01 Transitional, but it follows XML syntax rules. It supports everything found in XHTML 1.0 Strict, but also permits the use of a number of elements and attributes that are judged presentational, in order to ease the transition from HTML 3.2 and earlier. These include
XHTML 1.0 Frameset: is the same as HTML 4.01 Frameset, but it follows XML syntax rules. It allows the definition of an HTML frameset—a common practice early in the timeline of the mainstream Web.
XHTML 1.1
The most recent XHTML Recommendation by the W3C is XHTML 1.1: Module-based XHTML. This is a reformulation of XHTML 1.0 Strict, with minor modifications, using a set of modules selected from a larger set defined in Modularization of XHTML, which itself provides a modularized framework, a standard set of those modules, and various conformance definitions. All deprecated features of HTML, such as presentational elements and framesets, and lang and anchor name attributes, which were still allowed in XHTML 1.0 Strict, have been removed from this revision. Presentation is controlled purely by CSS. This version also allows for ruby markup support, needed for East-Asian languages (especially CJK).
Module-based XHTML adds the concept of a "strictly conforming" document: it must be a complete document containing only elements defined in the modules required by XHTML 1.1. For example, if a document is extended with the predefined XHTML Frames module, or some module formulated by a third-party, it doesn't qualify as strictly conforming XHTML 1.1 for use on the Web.
As of 2007, a second edition of XHTML 1.1 is in development as a W3C Working Draft.
Other members of the XHTML family derived from XHTML 1.1
XHTML Basic: A special "light" version of XHTML for devices that can not support the larger, richer XHTML dialects, intended for use in handhelds and mobile phones. This is the intended replacement for WML and C-HTML.
XHTML Mobile Profile: Based on XHTML Basic, this OMA (Open Mobile Alliance) effort targets hand phones specifically by adding mobile phone-specific elements to XHTML Basic.
XHTML+Voice: XHTML combined with Voice XML to support both visual and voice interactions.
XHTML 2.0
Work on XHTML 2.0 is, as of 2008, still ongoing. The current XHTML 2.0 Working Draft is controversial because it breaks backward compatibility with aspects of previous versions (X)HTML.
New features brought into the HTML family of markup languages by XHTML 2.0:
HTML forms will be replaced by XForms, an XML-based user input specification allowing forms to be displayed appropriately for different rendering devices.
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, for example,
Articles
, similar to XLink. However, XLink itself isn't compatible with XHTML due to design differences.
Any element will be able to reference alternative media with the src attribute, for example,
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, for example, 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've significant non-presentational uses and are required by certain languages. All other tags are meant to be semantic instead (for example 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.
HTML5 (Draft)
On 2007-04-10, the Mozilla Foundation, Apple and Opera Software proposed that the new HTML working group of the W3C adopt the WHATWG's HTML5 as the starting point of its work and name its future deliverable "HTML5". A month later, the new HTML working group resolved to do that. On 2008-01-22, the working group published HTML 5 as a Working Draft.
The HTML5 draft defines a parallel XML serialization for HTML5. The XML serialization is called XHTML5. Unlike XHTML 2.0, XHTML5 is intended to be compatible with XHTML 1.x.
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 doesn't 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.
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 can't locate one based on the public identifier (the other quoted string). It doesn't need to be the specific URL that's 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">
HTML 5
HTML5 doesn't require a doctype, and HTML 5 validation isn't DTD-based.
XHTML 2.0
XHTML 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">
A placeholder DTD schema exists at the corresponding URI, though it currently only includes the character reference entities from previous recommendations. XHTML 2 contemplates both a version attribute and an xsi:schemalocation attribute on the root HTML element that could possibly serve as a substitute for any DOCTYPE declaration.
XML namespaces and schemas
In addition to the DOCTYPE, all XHTML elements must be in the appropriate XML namespace for the version being used. This is usually done by declaring a default namespace on the root element using xmlns="namespace" as in the example below.
For XHTML 1.0, XHTML 1.1 and XHTML5, this is »<html xmlns="http://www.w3.org/1999/xhtml">
XHTML 2.0 requires both a namespace and an XML Schema instance declaration. These might be declared as »<html xmlns="http://www.w3.org/2002/06/xhtml2/"
/>xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.w3.org/2002/06/xhtml2/ http://www.w3.org/MarkUp/SCHEMA/xhtml2.xsd">
This example for XHTML 2.0 also demonstrates the use of multiple namespaces within a document. The first xmlns default namespace declaration indicates that elements and attributes whose names have no XML namespace prefix fall within the XHTML 2.0 namespace. The second namespace prefix declaration xmlns:xsi indicates that any elements or attributes prefixed with the xsi: refer to the XMLSchema-Instance namespace. Through this namespace mechanism XML documents allow the use of a mixture of elements and attributes taken from various XML vocabularies while avoiding the potential for clashes of naming between items from independently developed vocabularies.
Similar to the case of DOCTYPE above, the actual URL to the XML Schema file can be changed, as long as the Universal Resource Identifier (URI) before it (which indicates the XHTML 2.0 namespace) remains the same. The namespace URI is intended to be a persistent and universally unique identifier for the particular version of the specification. If treated as a URL, the actual content located at the site is of no significance.
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:
Failing to realize that documents won’t be treated as XHTML unless they're served with an appropriate XML MIME type
Not closing empty elements (elements without closing tags in HTML4)
Incorrect:
Correct: Note that any of these are acceptable in XHTML:
</br>, and <br />. Older HTML-only browsers interpreting it as HTML will generally accept 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
Incorrect: Cars & Trucks
Correct: Cars & Trucks
Using the ampersand outside of entities in URLs (Note that this would also be invalid in HTML)
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:
Correct:
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 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.
» :In the fully conformant application/xhtml+xml case, the non-XML code is wrapped in a CDATA section as follows (External Link):
)
HMS Audacious