ePub format is in essence a zip of HTML web files. Simple enough.
Still, most of ePub readers are not quite optimal...
I just randomly "discovered" that Edge can read ePub, quite well.
Microsoft Edge Browser Can Read EPUB eBooks
"Microsoft is going to be unveiling their first large scale digital bookstore sometime in the next few months. They are going to be populating it with over one million titles and will have a dedicated editorial staff to curate seasonal themes. Instead of developing an e-reading app, Microsoft is going to upgrade the EDGE internet browser with full support for EPUB."