RDF may occur only in the head section of the XHTML document and SVG and MathML only in some block or inline HTML elements within the document's body.
What needs to be done
Download XHTML DTDs and convert them to W3C XML Schema
Adjust the converted schema and create abstract definitions for head, inline and block context
Download schemas for RDF, SVG and MathML
In case those schemas are in a different schema language than W3C XML Schema, again we need to convert them first
Schemas for each language needs to be adjusted to make them part of the parent XHTML schema
Adjusted schemas need to be referenced from the head, inline and block abstract definitions within the XHTML schema