XHTML

A transitional format between HTML and XML. It's actually HTML tweaked to comply with XML 1.0.

v2 (proposed) is not backward compatible with v1.1

Problems on how to serve it:

Ian Hixie on the issue: If you use XHTML, you should deliver it with the application/xhtml+xml MIME type. If you do not do so, you should use HTML v4 instead of XHTML. The alternative, using XHTML but delivering it as text/html, causes numerous problems that are outlined below. Unfortunately, MsIE v6 does not support application/xhtml+xml (in fact, it does not support XHTML at all). oy I think I'll be sticking with HTML for the foreseeable future...

If you're using it on your website, even if it validates, you are probably using it incorrectly.

Bill Seitz