> An obvious issue is the lack of real machine number types This was basically what the special asm.js support in JS engines was about (via "use asm") and brou

> An obvious issue is the lack of real machine number types This was basically what the special asm.js support in JS engines was about (via "use asm") and brought performance to about WASM level. IIRC this special asm.js support brought a 2x to 3x performance advantage (e.g. running asm.js in Chrome/Firefox which both had special asm.js support vs Safari which didn't). AFAIK the initial WASM implementation in browsers was more or less an evolution of that asm.js special-path.