The html5lib conformance tests when combined with the WHATWG specs are even more powerful! I managed to build a typed version of this in OCaml in a few hours ( https://anil.recoil.org/notes/aoah-2025-15 ) yesterday, but I also left an agent building a pure OCaml HTML5 _validator_ last night.

This run has (just in the last hour) combined the html5lib expect tests with https://github.com/validator/validator/tree/main/tests (which are a complex mix of Java RELAX NG stylesheets and code) in order to build a low-dependency pure OCaml HTML5 validator with types and modules.

This feels like formal verification in reverse: we're starting from a scattered set of facts (the expect tests) and iterating towards more structured specifications, using functional languages like OCaml/Haskell as convenient executable pitstops while driving towards proof reconstruction in something like Lean.