XHTML: HTML for the New Era
To keep pace with the times, an "improved" version of HTML, called XHTML is finding its way into more and more Web sites. If you already understand HTML, don't worry—XHTML isn't a revolutionary new language that takes years to learn. It's basically HTML, but with stricter guidelines.
The hot debate's whether HTML or XHTML is the best approach. To judge by some of the online discussions, you'd think HTML and XHTML are completely different languages. They aren't. You can build snazzy and functional Web sites with HTML now and will probably be able to continue in the future. If you continue using HTML, the most important thing is that you follow the guidelines discussed in Chapter 1: in particular, an HTML page must use the correct "Doctype" (Section 1.3) or your CSS will fall apart in certain browsers, and it must "validate" (Section 1.2.4) so that you know for sure there aren't any typos or other mistakes that can mess up how your HTML displays.
You need to do those same things for XHTML, but because it's stricter than HTML it enforces rules that make sure the page works (for example, a Doctype isn't absolutely required in HTML; it is with XHTML).
XHTML's the future of Web page languages: there won't be any future versions of HTML, but there's a lot of work being put into creating the next generation of XHTML. (Don't hold your breath, though—it'll be years before there's wide support in browsers for it.)
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If you really want to delve into the innards of ...