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.

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

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