HTML: text/html
, full-stop.
XHTML: application/xhtml+xml
, or only if following HTML compatbility guidelines, text/html
. See the W3 Media Types Note.
XML: text/xml
, application/xml
(RFC 2376).
There are also many other media types based around XML, for example application/rss+xml
or image/svg+xml
. It's a safe bet that any unrecognised but registered ending in +xml
is XML-based. See the IANA list for registered media types ending in +xml
.
(For unregistered x-
types, all bets are off, but you'd hope +xml
would be respected.)