They don't have to: they are the ones creating the URLs! If they tack a
[XYZ] at the front or end of the URL (wherever your proposed standard puts
it), they should certainly be able to serve it.
This is an issue for people who create the URLs, and for the Web browser
writers. It doesn't have anything to do with HTTP or HTML (the two IETF WG
lists on which this seems to be being discussed). It's a URI issue all the
way.
--Paul Hoffman
--Proper Publishing