[tomsem]

I think Amazon's implementation of popup footnotes involves heuristics and is not too sensitive to how they were encoded (ePub 3 has specific attributes you're supposed to include). So in theory, it can improve even if the ebook itself does not change. But it is not perfect: I have a book of chess problems, the link to the solution invokes a popup but it only shows the text of the back-link, not the solution text. So I still have to jump to where the footnote is and navigate back with the back-link.

A typical convention is to provide a back-link with the footnote itself that takes you back to the reference. But with some reading systems, this triggers re-pagination, placing the reference at the start of the page when originally it was somewhere else. Some reading systems have a Back button that takes you back without re-pagination, and that works even if there is no back-link. But nothing is better than bringing it up in place.

I'm still curious as to whether this is one of the 'enhanced typography' features. At least on Kindle it pre-dated 'enhanced typography', but it didn't work with all books. I'll be trying stuff to see...