Adequately marked up <a href="..." epub:type="noteref"> footnotes (actually called endnotes) are rendered nicely in epub readers like the Kindle: clicking on on

Adequately marked up <a href="..." epub:type="noteref"> footnotes (actually called endnotes) are rendered nicely in epub readers like the Kindle: clicking on one opens it in a scrollable modal. I've seen some desktop ereader software render them as page footnotes. And since they are basic links, they work in a PDF and of course on a web page without any extra work on your part. Jumping isn't that great (and endnotes should always have a jumpback link) but it's not a bad baseline impl. I think following the endnote + noteref epub spec is the way to go. How they actually get rendered, like putting them inline, is then a trivial concern for the reader software. I personally like the modal overlay approach that some readers do. It handles endnotes of unlimited length and lets them be arbitrarily complicated since the modal is basically just an inset page. For example, a downside of inline footnotes is what should be done when they are too long to fit on the remainder of the page? Do you scroll next-page until you finish reading it, then prev-page back to where you started? etc. Either way, seems ideal to leave that concern to the reader software.