XHTML
XHTML is an official W3C recommendation. It defines an
XML-compatible version of HTML, or rather it redefines HTML as an XML
application instead of as an SGML application. Just looking at an
XHTML document, you might not even realize that there’s anything
different about it. It still uses the same <p>, <li>, <table>, <h1>, and other tags you’re familiar
with. Elements and attributes have the same, familiar names they have
in HTML. The syntax is still basically the same.
The difference is not so much what’s allowed but what’s not
allowed. <p> is a valid XHTML
tag, but <P> is not. <table border="0" width="515"> is legal XHTML; <table border=0 width=515> is not. A paragraph prefixed
with a <p> and suffixed with
a </p> is legal XHTML, but a
paragraph that omits the closing </p> tag is not. Most existing HTML
documents require substantial editing before they become well-formed and valid XHTML documents. However, once
they are valid XHTML documents, they are automatically valid XML
documents that can be manipulated with the same editors, parsers, and
other tools you use to work with any XML document.
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