Non-normative / not-implemented:
-- introduce numeric error codes
-- cycle detection in internal entities (and possibly external?)
-- conditional sections in DTD
-- validation of elements (regular expressions, non-cdata)
-- validation of attributes (unfinished)
-- notations
-- URI normalization
-- support for xml:space
-- support for xml:lang
-- full support for standalone documents
-- Unicode normalization

Optimizations:
-- detect definitions of trivial entities