As for the "namespaces aren't SGML" argument, it doesn't seem to be an
issue - we just have to settle on a "namespace prefix" and hardcode it
in the DTD.
You can actually do a little better than that. It can be
parameterized in such a fashion that the prefix can be changed in the
internal DTD subset. XHTML, MathML, SVG, and all the recent W3C recs
I gather from reading the various W3C standards documents that anyway
this has to be done in XML too if one wants valid documents, right ?