> HTML with MathML may make a decent system as well
HTML is fine to write by hand (especially when you take advantage of tag omission [0]), but I can't imagine handwriting MathML, since even simple equations need lots of tags [1].
[0]: https://html.spec.whatwg.org/multipage/syntax.html#syntax-ta...
[1]: https://en.wikipedia.org/wiki/MathML#Presentation_and_semant...