This project is from 2017. It is intended as a fallback and by nature not perfect.
There is NO javascript code! Github's detection seems to be broken somehow.
Have a look at index.html. You should see some nice mathematical expressions. Only Firefox and Safari will actually render MathML correctly, though. When a Chromium encounters MathML, it tries to render it as XML elements and looks awful. cmml.css needs to be loaded to style that "XML" in ways that look almost right. Due to limitations of CSS (such as no lookahead), not all features of MathML can be implemented. Height-scaling brackets are impossible. The root-symbol is an amalgamation of border and unicode-sign, beauty will depend on the font used.
There exist browser specific hacks to target Safari and Firefox. (Bonus: Chrome #24, the only Chrome to support MathML, is excluded!)
These queries need to be negated, so Firefox and Safari are excluded instead of targeted. I'm not sure weather this works reliably across all browsers, test on your own!
A CSS-file is loaded when MathML is not supported. The according media-query should be:
not \\0 screen or (-moz-images-in-menus:0)
(a not always negates the full media-query.)
Once media-query is cleared, the fallback-css is loaded.
There is a JS-based method of loading fallback-CSS, which looks at the aspect-ration of a fraction. Fred Wang uses this method, which should be easy enough to implement, but I wanted a CSS-only solution.
The fonts are all taken from MathJax. They are loaded inside local_fonts.css and need to be declared for the math element. (index.html uses MathJax_Size3.)
index.html contains a basic implementation and some formulas to test the CSS-fallback. If you use firefox, disable MathML in the about:config and remove the media-selector to test the fallback.
Due to it's very nature, this can't be tested automatically, as it needs to "look similar" in different browsers. (yes, visual AI, but that's too much overkill atm.)
Speed. JS takes time to load and I don't like that. So here is a completely CSS-based MathML fallback. I edited Fred Wang's original CSS a bit, mostly regarding roots.