I agree that people mis-use the “formal” word when it comes to specifications. But on this specific question, https://www.cl.cam.ac.uk/~caw77/papers/mechanisin

I agree that people mis-use the “formal” word when it comes to specifications. But on this specific question, https://www.cl.cam.ac.uk/~caw77/papers/mechanising-and-verifying-the-webassembly-specification.pdf https://www.cl.cam.ac.uk/~caw77/papers/mechanising-and-verif... is the work I am aware of.