I'm merging it, thanks! By the way, two observations:
There is currently no developer tutorial, but I plan to explain how to contribute on CONTRIBUTING.md next week.
Although the produced nodes are not exactly equivalent to KaTeX they produce the desired render, which is what's most important. These inaccuracies are rather a global codebase problem right now, and I plan to iron it out after doing some refactoring to simplify the logic of the code in general.
Implements serif, sans, frak, and mono.