In my own experiments with mathematical writing, rendering KaTeX server-side increased the resulting HTML size by a nontrivial amount. I bet that if you took something small-textbook-sized, in terms of bytes it would be smaller to ship katex.js and get it to render the latex on the fly, rather than generating the HTML ahead of time.

Bit of a sad situation, but then again HTML and CSS is not really the best layout language for this kind of stuff.