As an adjacent dead comment points out, you'd use Emscripten to "convert nginx to WASM" (i.e. "compile"). Wasmer is a runtime.
But there are actually use cases to what you're talking about. Essentially instead of nginx -> LLVM -> machine code you're replacing it with nginx -> WASM -> cmm_of_wasm -> machine code. There are tons of LLVM opts already, but if there weren't, it might be reasonable to use WASM as IR and using cmm_of_wasm's opts to make it faster than a naive C compiler (but of course none of the popular compilers are naive anyways). Also, of course this won't work today because cmm_of_wasm doesn't rebuild the syscalls from emscripten-compiled-WASM (yet). But if you're writing your own language today, ignoring system interaction for a sec, targeting WASM even for native might not be a bad idea as backends are coming w/ optimizations.
WASM currently does not have a GC but that's precisely why one is needed. Languages that target WASM need to implement their own GC but there's already a proposal to integrate a GC implementation into WASM[1].
As Wikipedia says of ml (meta language) languages in general:
"Its types and pattern matching make it well-suited and commonly used to operate on other formal languages, such as in compiler writing, automated theorem proving and formal verification."
Algebraic data structures are really nice for representing ASTs and intermediate languages. And a compiler lends itself very well to functional programming, because what you ultimately want is just a function that takes the input code and produces the output code.
