Note that we can't appeal to HTTP for validation of any of the
multipart formats -- they quite aggressively don't use them.
I run multipart over HTTP regularly for some HTTP/based remote file
processing systems. I also deployed multipart/mixed over HTTP as part
of a kiosk application that actually ran in stores all over canada.
In addition, a hack called multipart/x-mixed-replace was used for
early animation work by netscape.
But alas, I've never seen a multipart/alternative in this context.
-Rens