Some great responses here, thank you, along with some head-scratchers (e.g. use PDF instead)...
One reason I posted here is because I'm asked about Javascript in ebooks a lot, as I teach InDesign > EPUB and have a few video courses on it. I don't cover coding really, other than demo-ing Sigil and Calibre and BBEdit should the intrepid ebook designer want to try their hand at it.
The latest question from a user had to do with preventing the ereader from interpreting a tap on a button in an FXL as a tap to move to the next page. He reported back that he found the solution in the Kobo documentation on github, in the Javascript section, which apparently also works for Books app.
https://github.com/kobolabs/epub-spe...script-support
Sharing that here because I found it it interesting that Kobo covers it at all.