The MathML3 XSD Schema is a non normative grammar derived from the RelaxNG Schema presented in Appendix A of the MathML 3 specification.
This is the top level file. It includes several other files, corresponding to the constituent schema as described in appendix A. These files may be be browsed on line.
A zip file including all the separate xsd files.
This is the home of the MathML 2.0 XML Schema from the MathML Working Group.
All the details needed to use it should be covered in appendix A of MathML 2.0, 2nd edition
This schema is not normative. It is only provided as a tool to validate MathML instances, without any guarantee on the accuracy of the results. This version will be updated gradually to incorporate corrections or changes.
The schema files are available for download as a tgz archive, or can be browsed on line.