FWIW, while there are unfortunately only very few attempts at formalizing CSS, there's at least an unofficial, unreviewed (?), partial formal semantics for (CSS 2-era) float layout based on z3 SMT and Racket you can take a look at to get a flavor, though it isn't receiving further development.

[1]: https://github.com/uwplse/cassius

[2]: https://pavpanchekha.com/blog/css-floats.html

That would be amazing.

It would finally allow for competition in languages/paradigms.

And we could write modern languages from scratch.