After much work, the site is now XHTML 1.0 Strict compliant, and works with IE 7, Safari, Konqueror, Opera, and, of course, Firefox.

For browsers that support xhtml (Firefox and Opera), the xhtml is served as-is. For all others, it is rendered as HTML 4.01. It seemed the only way to get similar results on all of the tested browsers, and yet still be standards compliant.

I've added the W3C XHTML buttons to the bottom of the page.

One thing I still haven't tried is to serve XHTML to Safari. Currently, I'm sending HTML 4, because it uses (mostly) the same rendering engine as Konqueror, so I assumed that it would not handle XHTML. When I have a chance, I'll try it. If it works, I'll change the default for Safari.