Serve XHTML 1.1 Properly
Place this in a .htaccess file or your Apache configuration. Note that this will redirect noncompliant users to a chosen URL. All XHTML 1.1 pages must be saved with the .xhtml extension.
AddType application/xhtml+xml .xhtml <IfModule mod_security.c> SecFilterEngine On SecFilterScanOutput On SecFilterOutputMimeTypes "application/xhtml+xml" SecFilterDefaultAction "pass" SecFilterSelective "HTTP_ACCEPT" "!application/xhtml\+xml" "redirect:http://www.webstandards.org/upgrade/" IfModule>