This is great! Loved the demo and controls (and nice touch to include The Frog Prince as one of the sample texts lol). I think by doing tracking and expansion you're going even further than original Knuth TeX did/does. Maybe for the demo when hyphenation is turned off, the browser side should also turn it off? Or leave it like this, justif without hyphenation still does better than browser with hyphenation :)
Wish this project great success!