[amai]

Why use this instead of footnotes? For example in these Feynman lectures below the footnotes and references to formulas and images activate then you hover over it. These footnote can even include graphics and formulas.

https://www.feynmanlectures.caltech.edu/III_21.html

[StavrosK]

To me, footnotes serve a different purpose, e.g. linking to papers, like the Feynman lectures site does. Expounder is more about indicating that you don't know something, so the text itself can change to accommodate you.