XHTML reformulates the existing HTML technology to be an application of XML. It is already used for mobile phone and PDA Internet sites, and expected to gain widespread use as XML becomes more popular. Dan Wellman explains the differences between XHTML and HTML 4, and shows how easy it is to make the switch.
XHTML was invented to try to clean up the mess left by trying to make HTML a full-fledged presentation tool, and to reduce the fragmentation of HTML caused by the introduction of non-standard elements, mainly by Microsoft and Netscape. Additionally, XML is viewed by many as the future of the Internet, so reformulating existing markup technology to be an application of XML is a step towards embracing that future and letting go of the long standing legacy of the past. XHTML is now the specification of choice for mobile phone and PDA Internet sites, so its cross platform functionality is already being taken advantage of in excellent ways.
XHTML and its predecessor HTML 4 are extremely similar languages to use. Anyone that has worked with HTML 4 (or XML even) will find switching to XHTML extremely easy; it is more often then not simply learning to break those bad coding habits that many of us have found so easy to slip into. Remembering the few rules that come with XHTML, and trying to avoid any deprecated tags where possible, is all it really takes.
In HTML, the mark-up surrounding your content is traditionally referred to as a tag, the <a> tag for example. In XML and subsequently, XHTML, these tags and the text enclosed within them are known as elements. For example:
<p>This sentence, including the opening and closing tags is known as a paragraph element</p> <p>This is a separate paragraph element</p>