Using XHTML 2 schemas
The RELAX NG kind, and maybe the XSD kind.
I wanted to use Emacs+nxml to create some XHTML 2 documents, so I went looking for an XHTML 2 schema. The latest Working Draft says that it "includes an early implementation of XHTML 2.0 in RELAX NG, but does not include the implementations in DTD or XML Schema form. Those will be included in subsequent versions, once the content of this language stabilizes." This schema's location is not obvious, but a few web searches turned up a pointer to the ZIP archive version of the Working Draft mentioned in the

