I’m an happy Elm user; but I wonder if it’s a worth investment to start using Elm when we’re heading to a new ecosystem on top of WebAssembly. I use Go on backend, and would feel satisfied when Go and Webassembly works together fine.
On the other hand, Elm is not 1.0 yet. They are still breaking the ecosystem with new releases.
I also wouldn't be surprised if Elm compiled to wasm in the future.
AFAIK it's more about missing stuff like DOM access and GC that stands in the way.
asm.js is different enough that it's just not about adding a seperate asmjs "compatibility" target. On the top of my head you don't have 64-bit ints or floats and I believe the way you call asm.js fns from/to JS is different from wasm. Adding asm.js support just for IE 11 wouldn't be worth the effort.