We need a header element to reference or embed a schema or odd that the document is meant to validate against.
This should be able to cope with different namespaces... i.e. I should be able to reference the relaxNG schema for elements starting with rng: the tei: schema I'm using, the svg: schema I'm using, etc.
I should be able just to point to an ODD, an RNG, etc. or embed my ODD etc.