[GuiA]

Kudos to the developer for seeing a need and building something that fills that need with a clean UI. As a developer, I get that the quickest path to building a well featured eBook reader meant building on top of ePub.js.

However, purely from an idealistic engineering perspective (I know, some people do not care at all about that), having to rely on the DOM + a fully featured JavaScript engine to render an ebook is just... insane.

[PyroLagus]

Well, epub is just special HTML. I don't think you strictly need JavaScript, but it's about the easiest way to work with and render HTML. And I imagine rendering PDF is a whole lot messier.