Making those xhtml headers right

Apache can make it easy for you to give the right mime type for xhtml docs.

# server all files w/ ext .xhtml as application/xhtml+xml to those that can
# handle it
AddType  application/xhtml+xml xhtml

RewriteEngine On
RewriteCond %{HTTP_ACCEPT} !application/xhtml\+xml
RewriteCond %{HTTP_ACCEPT} (text/html|\*/\*)
RewriteCond %{REQUEST_FILENAME} .*\.xhtml
RewriteRule ^.*$ - "[T=text/html,L]"