[ptx]

But what if I want to target all the engines, including future ones? The compiler could compile separate versions for each engine, I guess, and you could choose which one to load at runtime based on the UserAgent string ... but even then everyone would have to recompile their websites every time a new browser version comes out.

The advantage of WebAssembly is supposed to be (I think) that it's simpler and will give more consistency between browsers, so browser behaviour will be less surprising, and you can thus get away with compiling a single version.

And if you take this approach of compiling JavaScript to a simple and consistent subset of JavaScript that can be optimized similarly in all engines, you'd end up more or less targeting asm.js, the predecessor to WebAssembly. :)