Interestingly, the same author was the first to create and publish an SGML DTD for WHATWG HTML (aka HTML 5) [1], though I believe mine [2] to be more complete and up-to-date at this point.

[1]: https://jkorpela.fi/html5-dtd.html

[2]: http://sgmljs.net/docs/w3c-html52-dtd.html