A simple system plugin that will check the HTTP_ACCEPT headers to see if the browser supports the application/xhtml+xml MIME type, and if so, sets the Content-Type header accordingly. Since IE doesn't recognise this MIME type, the plugin is useful only where you want to make use of XHTML-based technologies (e.g. XForms, SVG, MathML) for a limited (or controlled) set of users.

Read more at: http://extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,4721/Itemid,35/