application/xhtml+xml, or only if following HTML compatbility guidelines,
text/html. See the W3 Media Types Note.
application/xml (RFC 2376).
There are also many other media types based around XML, for example
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
x- types, all bets are off, but you’d hope
+xml would be respected.)