text/x-html-tables
without registering anything or agreeing with anyone, but
text/html-with-tables
isn't allowable unless the type is registered, and I would expect some
objections to its registration.
HTTP client/servers should be enhanced to allow version and level
attributes in its 'accept' and content-type attributions, e.g.,
text/html; level=3, version=2.0
may (or may not) be equivalent to:
text/sgml; dtd="-//IETF//DTD HTML Level 3//EN//3.0";
I was happy with the original plan that HTML actually have levels,
with the reasoning that browsers that can implement math could be
expected to also implement tables and forms. I'm not sure why you're
backing off from that position.
I don't like your coercing */* into application/octet-stream; the
content-type of the result matters.