I very much like Dan Connolly's suggestion of a time back:
Regard a media type as relative URL rooted at some agreed
place, probably iana.org so that the media type text/xml then
represents the URL
http://iana.org/text/xml
(or similar).
Now when we need an extra layer of fallback, it is obvious how to do
it:
text/xml/xhtml
sorry, it's not going to happen. the MIME content-type syntax is
well-established; changing it would break a huge amount of deployed software.
furthermore a two-level type system was an explicit design decision
of the 822ext working group. this decision was made for good and valid
reasons, and nothing which has happened since has demonstrated that
such a decision was shortsighted. in particular:
nobody has demonstrated a need for an extra level of type classification.
nobody has demonstrated a need for an extra level of fallback behavior.
if anything, experience with the existing two-level type system
indicates that fallback behaviors are of dubious value.
Keith