I wrote an epub reader a few years ago and this is false.
most epubs are just zipped html, but not all.
There are different versions to this file format and some need to be parsed as xml. The chapter files will mostly adhere to html with caveats wrt anchor tags, image sources and similar as well as metadata wont be parsed/work with html parsers.
Yes, but while thatd be a lot easier for epub (vs PDF), you can fundamentally do that for any other format too.
You could even - technically - take a picture as an input and then render it via background color rgb() using divs pixel-by-pixel. thatd still fall under that description.
most epubs are just zipped html, but not all.
There are different versions to this file format and some need to be parsed as xml. The chapter files will mostly adhere to html with caveats wrt anchor tags, image sources and similar as well as metadata wont be parsed/work with html parsers.