> > 
> > 1. Any XML parser should conform 100% with the XML rec:
> >   well-formedness, encodings, Unicode, etc.
> >   The OASIS test suite seems to be a good way to test this.
> >   This is very important, so that the Ruby XML tools can play with
> >   other XML tools (which is what XML mainly is about).
> 
> It is also important that DTDs are handled correctly. DTDs are not just for
> validation, for example, they define default values, certain types (e.g.
> ID), and parsed entities (there is something about this in the XML 1.0
> spec). These are all necessary even without validation.

Validation is part of the XML 1.0 spec.  A Ruby parser would need to be able to provide validation at the request of the user.