phw_DTD, a Textpattern CMS plugin
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.