There is of course sxml, which is more or less well-defined. But that would probably suffer from the same problems as xml, since it is just xml written as s-expressions.

There is one pretty damn solid SSAX parser by Kiselyov that has been ported to just about every scheme out there. It is interesting since it doesn't do the whole callback thing of most ssax parsers, but is implemented as a tree fold over the xml structure.