Getting arbitrary latex content to render well in HTML using mathjax is not trivial. There is a lot of valid tex that doesn't render correctly in mathjax without various little tweaks (adding newlines mainly) and of course this doesn't include package dependencies etc. I encounter this all the time because my personal notes in obsidian include a lot of equations etc. Mathjaxx will very frequently fail to render valid tex as html when you have something like $$\begin{align} and I have to go through and change this to

$$

\begin{align}

...to get it to work. On another site I try to post equations to, mathjaxx will stochastically mangle equations if I post them on my ipad vs working fine (except for the issue above) if I post from my laptop.

Yeah but pretty much no browser implemented mathml until fairly recent. A spec is worth very little without an usable implementation.

They didn't work well at all back then. And they still work worse than LaTeX, although the deficiencies have become acceptable.