Just seen Bug
MDL-7861 - I've been looking at something similar, initially for debugging. In DEBUG_DEVELOPER mode this patch delivers pages as the preferred 'application/xhtml+xml' mime-type to supporting browsers (Firefox, Opera, iCab... Safari issue with character entities, http://www.w3.org/2000/07/8378/xhtml/entities) and 'application/xml' for Win-IE 5+ (http://www.w3.org/MarkUp/2004/xhtml-faq#ie). Other browsers get 'text/html'.
An XML well-formed bug (stray &, unquoted attributes...) leads to an XHTML browser displaying an error message in place of the page, which should be useful for developers. (Note, an on/off switch may be useful to aid diagnosis.)
The patch also substitutes the XHTML Strict document-type declaration in DEBUG_NORMAL mode - note, error messages aren't displayed in the browser, a 3rd party validator is required.