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Extensible Hypertext Markup Language (XHTML) is a markup language that has the same expressive possibilites as HTML,
but a stricter syntax. Whereas HTML was an application of SGML, a very flexible markup language,
XHTML is an application of XML which is a subset of SGML. XHTML 1.0 became a
World Wide Web Consortium (W3C) Recommendation on
January 26, 2000.
XHTML is the successor to and the current version of HTML. The need for a more strict version of HTML was felt primarily as
now web content needs to be delivered to many other devices (like mobile devices) apart from traditional
computers, where extra resources cannot be devoted to support the generosities of
HTML (the looser the syntax of a language, the harder it is to process). The XHTML
DTD is defined within the XML DTD to enforce the strict rules of XML.
Most of the recent versions of popular web browsers render XHTML
properly, and many older browsers will also render XHTML as it is mostly a subset of HTML and most browsers do not require valid
HTML. Similarly, almost all web browsers that are compatible with XHTML also render HTML properly. Some say this is slowing the
switch from HTML to XHTML.
XHTML's true power is realized when used in conjunction with Cascading Style Sheets. This makes the separation of content and form an integral part of the web
page's code.
The changes from HTML, to transitional XHTML, are minor, and are mostly just in conformance with XML. The most important
change is the requirement that all tags are well-formed. Additionally, in XHTML, all elements must be lowercased. This is in direct contrast to
established traditions which began around the time of HTML 2.0, when most people preferred uppercased tags. In XHTML, all
attributes, even numerical ones, must be quoted. (This was mandatory in HTML as well, but often ignored.) All tags must also be
closed, even the empty tags <img> and <br>. This can be done by adding a closing slash to the tag:
<img /> and <br />. Attribute minimization (e.g., <option selected>) is also
prohibited. More differences are detailed at the W3C XHTML specification .
Wikipedia pages currently use valid XHTML 1.0 Transitional.
Versions of XHTML
XHTML is HTML reformulated as XML. There are three different versions, equal in scope to
their respective HTML4.0 versions.
- XHTML 1.0 Transitional: Intended for easy migration from HTML3.2, or for those using link targets or
inline-frames
- XHTML 1.0 Strict: Separates content from layout (which now moves to CSS)
- XHTML 1.0 Frameset: For splitting the browser window into several frames
- XHTML 1.1: Module-based XHTML; authors can import additional features (such as framesets) into their markup. This
version also allows for ruby markup support, needed for Far-Eastern
languages.
- XHTML Basic: A special "light" version of XHTML for devices which cannot
use the full XHTML set, primarily used on handhelds such as mobile phones.
This is the intended replacement for WML and
C-HTML.
- XHTML Mobile Profile: Based on XHTML Basic, this Nokia effort targets hand phones
specifically by adding mobile phone-specific elements to XHTML Basic
Work on XHTML 2.0 is, as of 2004, still underway. The XHTML 2.0 draft is
controversial because it breaks backwards compatibility with all previous versions, and is therefore in effect a new markup
language inspired by (X)HTML rather than a true successor of it.
External links
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