XML in a Nutshell, 3rd Edition

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.