XHTML is based on XML, and thus requires the source to be well-formed.
Since XHTML is more strict than HTML, less pre-processing is needed by the rendering engine.
XHTML should be served as application/xhtml+xml for you to take advantage of the benefits, otherwise XHTML will be treated as ordinary HTML. Serving it as ‘application/xhtml+xml’ is not common on the web due to Internet Explorer, which cannot handle XHTML.