What is it?
Extensible Hypertext Markup Language (XHTML) is essentially HTML extended and reformulated in XML. Like its predecessor, XHTML is a general purpose markup language for representing documents on the web. Of XHTML, the World Wide Web Consortium (W3C) said, "It does not attempt to be all things to all people, supplying every possible markup idiom, but supplies a generally useful set of elements, with the possibility of extension."
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Where did it originate?
HTML began life in 1995 as the W3C's answer to incompatibility problems between different suppliers' browsers. It is based on SGML (Standard Generalised Markup Language) but is much simpler. XML is also an offspring of SGML; it keeps the richness of the language while making it simpler to use.
XHTML 1.0 became a W3C recommendation in January 2000, and work on XHTML2 is well advanced.
What is it for?
There are different versions, including XHTML 1.0 Transitional, for easy migration from HTML; XHTML 1.0 Strict, which separates content from layout; and XHTML 1.1, or module-based XHTML, which enables authors to import additional features into their markup. For devices such as mobile phones, which cannot use the full XHTML set, there is a "light" version, XHTML Basic, the intended replacement for Wireless Markup Language.
XHTML2 is being made as close to XML as possible. Among the W3C design guidelines are, "If a facility exists in XML, try to use that rather than duplicating it." It is being made more device-independent, so documents will only need to be "authored" once to be displayed on telephones, PDAs, tablet PCs and televisions as well as desktop systems.
What makes it special?
XHTML can mix and match with other XML languages and use XML tools such as XSLT for transforming documents. Semantic Web applications will be able to take advantage of XHTML documents.
HTML was originally designed to represent the structure of a document, and presentation elements were added later. XHTML2 will remove them, handing presentation over to style sheets. "This gives greater flexibility, greater accessibility, more device independence, and more powerful presentation possibilities, since style sheets can do more than the presentational elements of HTML ever did," said the W3C.
How difficult is it to master?
The W3C recommends that beginners learn XHTML rather than HTML. Experienced HTML authors are said to be able to pick it up in their spare time in about a week. You can roll over your old HTML documents into XHTML using an open source HTML Tidy utility, available from the W3C website.
Where is it used?
HTML was known as "the lingua franca of the web". XHTML will replace it as the benefits promised from XML become widely appreciated.
What's coming up?
Areas that have received particular attention in XHTML2 include better structuring possibilities, removing features that are duplicated in XML, usability, accessibility, internationalisation, device independence, better forms and reducing the need for scripting.
Rates of pay
HTML vacancies still greatly outnumber XHTML positions, but XHTML with Cascading Style Sheets can earn £30,000 or more. HTML designers get from £18,000 to £28,000.
Most major training companies and software and hardware suppliers offer XHTML training, but there are some excellent free tutorials. Start with the W3C website, where you can see the latest progress on XHTML2. W3C HTML guru Dave Raggett has posted tutorials, and also written guidebooks.