Currently the schema only supports xml:lang on the the <o> element. The alternative is to support xml:lang everywhere, to allow it to be inherited, but to simplify processing, I’m avoiding this at the moment.

xml:base is not supported anywhere in the schema. I think that this might be the right decision. xml:base is tricky to support in XSLT 1.0 (although I have implemented support for it as part of my Atom/RDF project).