Even though we're using CSS as much as possible, we do need the character metrics in order to do positioning properly. (For example, $x^2$ and $F^2$ differ in the vertical position of the 2.) You'll need to update these metrics to use it with a different font -- see

https://github.com/Khan/KaTeX/blob/master/src/fontMetrics.js

and the "make metrics" Makefile rule. We haven't added the ability to swap font metrics at runtime or to easily build a version with different metrics but I'm not opposed to the idea.