Yes this is a thing and it’s been around for quite some time. If you’re trying to approximate TeX, you may also be interested in MathML.

I need to go lay down; I’m having flashbacks to the good old days of “XML everywhere for everything all the time”.

Try turning the phone to landscape mode before tapping the full screen button. It works for me in YouTube on Pixel 7a.

What are some good alternatives if it gets shut down? Preferably FOSS.