Not much to say about the complex implementation itself; seems pretty good, and it's as 'bare-metal' as you can get in browser javascript. It's not a great abstraction, and very little error checking.

However, I really like this literate programming style. You've got live editing, and legible output, in the form of little TeX blocks. It's a really nice way to define, describe, and teach little libraries like this. Well done.