XML itself is textual and we don't have any internal support for DTD or Schema as it is so I'm not sure what material benefit we gain by restraining ourselves here.
This apparently isn't true - the XML output representations include schema-verifiable namespaces and thus have a well-defined allowable set of output values and formats. While not every use case may have or need these at present their presence is embedded into the implementation - making any desired change non-trivial to evaluate.