Fotiman - 2:42 am on Feb 3, 2013 (gmt 0)

If your XHTML page is served from the server with a mime type of text/html, then then you're not really serving XHTML, you're serving HTML. If your server is configured to serve the page with a mime type of application/xml+xhtml, then browsers (that support it) will treat it as XHTML.

You should read the thread Why most of us should NOT use XHTML [webmasterworld.com]. It's almost 7 years old, but it's still referenced often. :)