David Durand writes:
> But SYSTEM IDs should not be included. In SGML, they are reserved for
> "non-portable" identifiers,
No. They are system specific. They are not _required_ to be non-portable.
SGML makes no promise that they are portable. but certainly does not say
that any document using a portable SYSTEM identiifier is invalid!!!
For XML, it would be possible and acceptable to requre specific forms of
system identifiers in certain places, and in fact we have already done so
by using URLs.