Textpattern CMS is a pretty robust publishing system that outputs valid xhtml by default. phw_DTD is a Textpattern CMS plugin that appends a XHTML1.1 DTD to pages and serves the documents with a mime-type of application/xhtml+xml to browsers that support it (WebKit based browsers, Gecko based browsers, Opera, W3C validator). Other, lesser browsers (I’m of course looking at you, Internet explorer) just get text/html with a XHTML 1.0 strict DTD. This plugin is based on the code developed by Simon Jessey.
A Readme file detailing usage is included. Note that to use this plugin, you’ll need to modify slightly one of the core Textpattern CMS files, as explained in the Readme.
This plugin is obsolete. Anno 2015, serving pages as pure XHTML is so old-fashioned (I doubt it still works…)