Thanks for that. I can't read the article, probably because I block WASM (and Javascript) for security. None of my ebook readers have Internet access (for security and for privacy), so none of those internet-only epub files would work for me.

This might be "legal", since XHTML was intended for the web, but I assume Google's using it to collect more user interaction data that they can sell to data brokers.

FWIW, PDF is simply Postscript that's been compressed. As far as I can tell, almost all documents these days are created with Microsoft Word, TeX, or Postscript. I'm lumping things like PageMaker and LaTeX in with the base they were derived from.