Not the OP, but I find Calibre burdensomely heavy for what I want, and Okular far too glitchy. Also, sometimes I am at PCs where I can't install arbitrary software.

I think we still need far more/better solutions in the open source e-reading space.

I can't speak for the creator but I often read Japanese ebooks in my browser because it allows me to lookup unfamiliar words quickly with Yomichan and then create anki cards for them (if I want to).

A bit OOT but how do you transfer Yomichan to anki cards? Do you do it manually?

It's a built in feature using Ankiconnect.

Have you tried calibre? It’s a proper ebook reader app that allows you to define custom dictionary URL patterns.

I use Calibre daily, but for epub > dictionary > anki card there isn't a good workflow I've found.

> multiple free desktop and mobile book readers exist

Under many circumstances, some I've been in, none of those are an option, and a browser is all you've got.

>> when multiple free desktop and mobile book readers exist

Do you know a good list of available readers? I still searching for the one which fit for me. This one could be a good candidate after some polish.