XHTML

The XHTML 2.0 draft specification

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Work on XHTML 2.0 is, as of 2007, still ongoing. The current XHTML 2.0 Working Draft is controversial because it breaks backward compatibility with all previous versions, and is therefore, in effect, a new markup language created to circumvent (X)HTML's limitations rather than being simply a new version. Many issues with compatibility are easily addressed, however, XHTML 2.0 can currently be parsed the same way a user agent would parse XHTML 1.1: via an XML parser and a default CSS document conforming to the current XHTML 2.0 Working Draft.
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.
• 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

London Bridge

.
• 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 will be indicated by the nested section elements, each with their own h heading.
• 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. strong 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.

Versions of XHTML

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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 follows XML syntax rules.
• XHTML 1.0 Transitional is the same as HTML 4.01 Transitional, but 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 center, u, strike, and applet.
• XHTML 1.0 Frameset: is the same as HTML 4.01 Frameset, but follows XML syntax rules. It allows the defining of an HTML frameset, a common practice in versions of HTML prior to HTML 4.

Differences from HTML

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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 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, and hence in HTML, where numeric or boolean attributes can dispense with quotes (quoted attributes are assumed to be strings). 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. Attribute minimization (e.g., option selected) is also prohibited as the attribute “selected” contains no explicit value; instead, use 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 element unless the author specifies one. More differences are detailed in the W3C XHTML 1.0 recommendation. Differences between HTML and XHTML from the (X)HTML5 point of view are detailed on the WHATWG wiki.

Beginning

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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.

Whereas HTML is an application of 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.