Sigil VS epubjs-reader

As for editing your ePub file after compile, I definitely recommend either Calibre or Sigil. Either of these will be integral to a publication workflow, even if you get most of the job done with Scrivener. There is no better way to figure out what goes where or what an error message means if your book doesn't pass validation (and will help with validation itself so you don't run into that situation in the first place).

Gihub: https://github.com/Sigil-Ebook/Sigil

I recommend using Sigil for this kind of work, as you can see the changes you make to CSS in real time, and more easily find the exact element class names you'll need to work with.

If it reads like gibberish to you though, try using Sigil or this thing if it even works.

How to use: https://standardebooks.org/contribute/producing-an-ebook-ste...

The second is Sigil, which is a great editor if you prefer to work with a GUI:

GitHub: https://github.com/Sigil-Ebook/Sigil

Download Calibre. Format your manuscript in Word how you want it to look and import it into Calibre to create an ePub file. Calibre does a very good job of translating your Word formatting into the HTML required for the ePUB file. If you want to do some tidying up to the ePUB file, download and use Sigil, which is a very good tool for editing ePub files. Both software tools are free.

There's a few JS libraries that allows you to basically have the same functionality, example: https://github.com/futurepress/epubjs-reader/