MathML is here today and works in Firefox. I use it on Wikipedia[0], which is the only major ("non-niche"[1]) website that provides it, and it's much nicer than the image-based equations (and much, much faster than MathJax would be).

[0] https://www.mediawiki.org/wiki/Extension:Math/advancedSettin...

[1] not that "niche" websites like nLab[2] should be disregarded, since the web was originally designed to help scientists...

[2] https://ncatlab.org/nlab/show/HomePage