Many books/websites let you view the Latex code, despite not technically being open source. If a website uses MathJax, you can change the renderer to "plain source" by opening the context menu on any math equation. "Paul's Math notes"[1] is a good example, and a really great free website for college-level math.

If you're a screen reader user, the context menu may not open for reasons unknown. Try routing your mouse to the equation and simulate a right mouse click, that should do it. Moving your focus to the equation with the tab key may also be an option. Alternatively, you can just use Voice Over for MacOS, it can do it just fine.