> if you just change the file extension of a ePub to html it will work fine in the browser If that's the case, you didn't have a genuine EPUB to start with. To

> if you just change the file extension of a ePub to html it will work fine in the browser If that's the case, you didn't have a genuine EPUB to start with. To meet the spec it needs to be in a container (a renamed ZIP file) and have a handful of related metadata and navigation files alongside it. That said, the actual text of the book is done by HTML/CSS, but within the EPUB container file.