I think it's a bit more than that given that the project has amassed more than 16.000 commits so far.

You must not forget that JS does not provide support for many of the things that are required to run a C program (e.g. printing), so emscripten needs to take care of providing alternative implementations for all these things.

Oh, alright. We are speaking about two different things. I was thinking about compilation and specifically supporting complex language features which are mostly handled by a compiler frontend which is this case is clang. And it seems that you are thinking about runtime libraries (or the whole tool chain).

Ah yes you're right, that part is luckily covered by LLVM.