The MusicXML W3C XML Schema Definition (XSD) is the preferred method for validating MusicXML documents. It provides a stricter definition of the MusicXML 4.0 format than does the MusicXML Document Type Definition (DTD).
The XSD improves the experience of MusicXML users by making MusicXML software more reliable. It also makes MusicXML easier to use with the latest XML technologies, including XQuery, XML databases, and XML code generators. The DTDs are deprecated as of MusicXML 4.0.
You can download all the MusicXML XSDs and DTDs as a single zip file from the W3C GitHub repository:
Each schema is also documented in the Community Group Report: