Here's my view:
The DTD will have a declaration:
<!ENTITY #DEFAULT SDATA "&unkown;" --display the markup-->
which says (in SGML) that arbitrary entity references are legal and
(in application conventions) that if you see an entity that you don't
recognize, just display the markup. So you might see:
When α > β, it is clear that...
So in the future, &pagebreak; won't be silently ignored -- it'll
either be handled in some way specified by the DTD, or it'll be displayed
as &pagebreak;.
On the other hand, <? pagebreak> should _not_ be displayed. The fact
that Mosaic fails to recognize this as markup is just a plain old bug.
The question of how to manage the namespace of processing instructions
remains. I suggest we don't manage it at all -- we leave it wide open
for private mechanisms (text formatting systems, index generators, and
the like). This leaves the possibility that private mechanisms will
clash, but ...
Dan