Three Web developers were sitting around arguing over what the most abundant thing on the Internet is.

The first one said, “It must be free music.”

The second one said, “Nahh…it’s got to be spam e-mail.”

The third one said, “It has to be porn.”

About that time, a Linux user overheard the conversation and decided to throw in his two cents. “All three of you are wrong. The most abundant thing on the Internet is bad code.”

XHTML, when created with the aid of a decent validation tool, helps enforce a stricter set of coding standards than most Web pages adhere to. The following can be said about an XHTML document: it is well-formed, it is both HTML and XML at the same time; and it is an HTML document that adheres to XML syntax rules.

To be valid, an XHTML document must have: