A quick HTML-based ePub Reader that works offline once you load the page online once (like open it and wait for the service worker to cache the dependancies).
This is a single .html file ebook reader. Download the standalone file in the Releases tab. A lot of stuff is questionably coded and this is only for ePub books.
Also, you can check it out at https://nellowtcs.github.io/HTMLReader/ if you don't want to run it from the HTML.