Have any of the ebooks included any serious math? I see that MathML is the expectation, but I wonder if the rules around the math aren't just ignored because of the rest of the culture around which books are selected and worked on.

Indeed, in academic writing I never see MathML used in an HMTL setting, it's always MathJax or KaTeX. For your purposes this is probably fine, but imagining if someone wanted to author a high quality math textbook following the same standard I would wager they'd run into a brick wall