Serving XHTML as application/xhtml+xml might not be
a good idea under all circumstances. DOM changes the behavior
(innerHTML often does not work, and neither does
document.write; Element/Attribute names are not
normalized to upper-case; document.createElement does not
work, only createElementNS and so on), CSS is applied
slightly different, <script> and
<style> blocks must use CDATA sections
instead of HTML comments, Mozilla Gecko will not render documents
incrementally etc.
For more information on application/xhtml+xml vs
text/html, read the following documents: