Yeah. Target asm.js and WebGL and you'll get Internet Explorer 11, Chrome, Firefox, IE and probably eventually Safari. With NaCl you get Chrome only. The choice is simple.
asm.js is supported, inherently, by all modern browsers. It is just JavaScript, after all. However, only Firefox has a second compiler path especially for it. Chrome has some optimisations for asm.js-like code in V8.
Well, yes, but I find the WebGL port of the Unreal Engine demo (for example) unplayable without asm.js-specific optimizations. So by "supported" I really mean "optimized".
