But, as has been mentioned, we have shied away from requiring UAs to
read an internal subset, because then authors can put anything in
there (ISO 12083, etc). So this is not the best way forward.
I don't agree. First, a document containing an internal subset is not
currently unconformant. Therefore UAs should be prepared to deal with
internal subsets now. The question is how much.
We can establish an HTML application convention that sanctions the
interpretation of only certain declarations and ignores others. I would
think this is a much preferable route to follow than inventing new
mechanisms and new incompatibilities. If SGML provides the mechanism,
then we should use it if at all possible rather than inventing a new
mechanism.
Glenn