In collaboration with @jpolitz, we've been working on way to debug programs written in arbitrary source languages in the browser that you may find interesting. You can find a technology demo here:
Stopify uses existing compilers that produce JavaScript (e.g., ScalaJS, BuckleScript, etc.) We've tested with ten languages, five of which are on the website. There are three key pieces to Stopify: (1) first-class continuations for JavaScript (warts included), (2) browser-specific optimizations, and (3) language-specific optimizations (i.e., many compilers emit a sane subset of JavaScript that Stopify can exploit to improve performance). We've conducted a pretty extensive performance evaluation that we can also share. E.g., we've found that PyJs + Stopify can be as fast as Skulpt with suspensions.