(Someone else has addressed the SGML mechanisms (which exist and
don't work))
If it doesn't work it is only because an HTML user agent is lacking or is
simply non-conformant; it is not because SGML is lacking. Since we are
talking about extensions for the future (and not simply modifications to
the HTML 2.0 DTD), then we can rightly consider requiring user agents to
become more capable w.r.t. SGML.
Glenn