The MusicXML Document Type Definition (DTD) was the original technology for defining MusicXML files. The DTDs have since been superseded by the W3C XML Schema Definition (XSD) and are deprecated as of MusicXML 4.0.
The DTDs are divided into several files as listed here. The Partwise and Timewise files are the top-level DTDs for musical scores. The Opus file represents collections of works. The Container file is used with compressed MusicXML files, and the container.dtd file contains information about the compressed file format. The sounds file maps between MusicXML sound IDs and application or library sound IDs.
You can download all the MusicXML DTDs and XSDs as a single zip file from the W3C GitHub repository:
You can also view the individual DTD files on GitHub:
The DTD for representing Standard MIDI Files in XML are also deprecated as of MusicXML 4.0.