I've changed my stance on this issue somewhat. That's because I hadn't realised how much I used the MS Edge EPUB viewer until MS took it away Ψ³
What did I like about it - very fast to load an epub -- and in no particular order and incomplete: not a librarian, just an epub viewer, TTS, annotations and highlighting, elegant (IMO) user interface, fairly decent search, features to assist language learning (don't know if it did non-English), IIRC that included: syllable break down, spell checking, dictionary lookup, and lightweight grammar checking.
I didn't really like the fact it was embedded within a full blown web browser, but it did mean I could view multiple epubs in a tabbed interface. I don't use Edge as a web browser very often so this wasn't a serious issue for me.
So, if Kevin wants a model for developing an epub reader, I'm saying the Edge viewer had all the features I want, and just as importantly none of those I don't want
Some say the reason they pulled the plug on ePubs in Edge was that it was too hard to implement what they did under the webkit-like Chakra layout engine under the Blink layout engine. Is that an echo of PageEdit I hear.
If a Sigil Viewer were developed I wouldn't object if it didn't use Qt
BR