Neat, once again, emscripten proves useful. I do find it important though to point out the lack of a good open text-to-speech engine.
Here is a speech as rendered by tts-api.com (http://goo.gl/PoZc4). Now, for speak.js [1], to make a comparison, paste in the first few of the top paragraphs from here [2] and compare the quality between the two.
There really is a gap to fill for a good open-source alternative here. But I suspect the main barrier is that there is a large amount of data needed to generate good voices. Still, a worthy target.
[1]: I tried to make a URL for this too, but despite the URL looking as if it could take arguments it refused to work, at least for me under Firefox and Chrome.
If you're looking for a client side solution, here's espeak compiled to JS using emscripten.