Ever wondered where all the HTML tags we use are defined? Why there is a <p> tag instead of <para>? And why a <table> tag requires a <tr> tag, which requires a <td> tag? Have you ever wondered what that DOCTYPE line is at the top of an HTML file? If you have a hankering for understanding DTDs, this one is for you.

Probably the best reason I have found to use XHTML is when your source code is XML and your output will be HTML. If you will be transforming XML into HTML, you are probably spending way too much time creating and coding in a custom XML language created by you or someone in your group. XHTML, not custom XML, or a hybrid of custom XML/XHTML is what you should be using for your source code.