If serving XHTML as HTML really offends some people’s sensibilities (apparently it does), then one way around it is the way I do it for part of my site:
In a .htaccess in the doc_root of your blog
RewriteEngine on
RewriteCond %{HTTP_ACCEPT} application/xhtml\+xml
RewriteCond %{HTTP_ACCEPT} !application/xhtml\+xml\s*;\s*q=0
RewriteCond %{REQUEST_URI} \.php$
RewriteCond %{THE_REQUEST} HTTP/1\.1
RewriteRule .* – “[T=application/xhtml+xml; charset=utf-8]”
This serves it is application/xhtml+xml to any user agent that supports that MIME type (nearly everything made in the last 6 or 7 years but IE) and leaves it as the page default, text/html, to those that don’t.
As I said, it’s a major calamity to some people out there to see XHTML served as text/html and the “solution” I’ve provided should help out.