1) it is undefined
2) use SGML processing and return the default entity if defined, null if not.
3) break with SGML and return the entity name.
Away from my desk, but I don't think it's defined in the spec (someone
will shout if that's wrong). I assume you mean a character entity like
&zogonek;, not an unresolved parameter entity. I think current
practice is just to display the unresolved entity name in full,
including ampersand and (hopefully) semicolon (like most browsers
return  : still).
I know what I'd _like_ to see browsers doing, but that's not a matter
for this list.
///Peter