OMITTAG specifications in element declarations

In XML, we've tossed out OMITTAG.  However, there's a question about 
whether we allow the "omitted tag minimization" parameter in XML 
element declarations.

I feel we should, so that existing DTDs that provide these specifications
don't need to be maintained in two separate versions, one for SGML and
one for XML.  I see no harm in allowing them.  In private conversation, 
Tim has said that we shouldn't, because it requires more cruft in the 
XML spec.