PreTeXtBook
EPUB MathJax SVGs sometimes render as black boxes
Some SVGs produced by MathJax for EPUBs end up rendering as black boxes (at least in Calibre).
See Sample Article, 7.6:
Yes, I did happen to see that one. I'm going to guess one of those symbols is causing problems (the Maltese cross?). I'll see about removing them one-by-one.
Its the hash (#) symbol.
Replacing the bare character (U+0023) by \# fixes the math, but then breaks the cross-reference.
I think I recently discussed exactly this with the MathJax folks. I'll find a reference later...
See: https://groups.google.com/g/mathjax-users/c/7kuF0TWIKOk/m/6n81gTkXAgAJ
Which got tangled up in another thread, and which has one small obvious typo from Davide.
In short, a typical LaTeX mess from over-zealous markup, and perhaps saved for v4.