I think Latexbase does something like this (they claim it works offline too, using service workers). They claim to be using Emscripten/LLVM but it doesn't seem like they use wasm yet (probably because they didn't consider it a viable option back in 2016 https://latexbase.com/p/b0a174a5-09b3-4598-9686-3a73be2dc8e5). And while it's not open source I still think it's a really cool proof of concept..