> Most compilers that target WebAssembly can also target asm.js, which is compatible with IE.
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.