Nb this doesn't appear to be a validator but merely a frontend to various backends that perform actual validation, such as W3C's classic SGML-based validator or nu validator for HTML, as can be seen in the sparse commit history.

Or, if it did perform validation, then only for the relatively simple case of XML DTD/XML Schema validation coming with Java/Xerces, which however has been irrelevant on the web for a long time now in the post-XHTML era.

Especially with W3C not bothering to publish schemas or other formal grammars in those areas they're still relevant for, even when it would be possible, useful, and was explicitly requested by the community, such as for SVG 2, or ARIA 1.2+.