After wrestling with XSL, HTML, and XHTML for weeks, I finally decided to simply create a Nexista extension which uses the Tidy library to fixup the output buffer as either HTML or XHTML. So far, its working great. I'm using it with PhunkyBB and its working fine. Hopefully this will make it a lot easier to switch between plain HTML 4 and XHTML output without having to edit any stylesheets!