[jeromie]

The team at ruffle.rs has done a really good clean-room implementation of Flash. The hard part with Flash (beyond having two virtual machines, a JIT compiler, pixel shader support, etc) was that it was also bug-for-bug backwards compatible, such that new player updates would not break existing content compiled for older versions. Good luck reverse-engineering that (or porting it with a fully copy of the ~2.5M line codebase).

It's also the case that a lot of games were dependent on specific timing, and changes to internals that were completely unrelated would elicit super weird behaviors in various games.

When Flash in the US was EOLed, the folks at Newgrounds used Ruffle to keep their large body of Flash content running. It's been a couple years since I've checked in on it, but what they had was pretty impressive compared to other FOSS attempts that had been made over the years.