[Ballot comment]
Not a blocking comment, but I note that the authors are embedding version information in XML content here.  An optional parameter containing that version information might make it easier
for an agent to know whether or not a document is from a supported version (given that interopability is not guaranteed across versions).  If this has been considered an rejected,
no problem, but it does seem worth mentioning as an optional parameter candidate for this
case.

Security considerations section of the MIME registration needs work. In particular, it needs to state whether or not this type employs any sort of active/executable content, and if it does, what steps are taken to insure it can be dealt with safely.

Document also needs to have a security considerations
section of its own. It can simply refer to the
security considerations section given in the
registration form.

References also need to be split into normative
and informative groups.

The IPR boilerplate called for by RFC 2026 section 10.4
is missing and needs to be added.