If the emscriptened code does the right thing, then its output is object code, no more in need of by-hand maintenance than the object code from a compiler, right?

Anyway, I'd be interested to see how much faster you are (at comparable image quality) than one of the suggested libraries. If you are in fact a lot faster, I'd be curious to know what it is that you're doing that you think makes it run significantly faster in current JavaScript engines than translated code.