You need the server side to do this. You can set the mime type but by then the browser is already rendering the page!@
If you had taken the time to actually read hte first page of this thread where Tommy goes over this, then you'd know.
This is taken frrom page 1
You need to instruct your web server to send the proper HTTP header. The exact technique depends on which HTTP server you use, but for Apache, you can use the AddType directive:
AddType application/xhtml+xml .xhtml .xht
This makes Apache send an application/xhtml+xml MIME type for files ending with .xhtml or .xht. The directive can be put in the global configuration file (/etc/httpd.conf on most *nix systems) or in a local .htaccess file in a directory.
Sometimes we don't have access to the configuration files, but we need not lose hope quite yet. If we can use a server-side scripting language, we can send the HTTP header ourselves. For instance, using PHP:
header('Content-Type: application/xhtml+xml; charset=utf-8');
(Note that this header must be sent before a single byte of document content is written to the response stream.)