Try Thorium. Still a work in progress, but they are trying to be up-to-date on standards compliance, which puts them ahead of a lot of reading systems. The UI is decent.

(you'd think maintaining an epub extension for a web browser wouldn't be that much of a stretch, considering that epubs are built on web tech, but who knows why they cut that.)

Thorium is cool! It's open source (BSD 3) and has support for LCP to read borrowed books without the Adobe Digital Editions app (The archive is currently still working to make it available as an option though. See https://help.archive.org/help/disability-access-general-info...)

I make do with SumatraPDF. https://github.com/sumatrapdfreader/sumatrapdf

KDE's okular might be a good choice. I haven't personally used it for epub but I know it supports it.

https://okular.kde.org/

Okular's epub rendering is awful. Nearly any other option will look better.

I live in Calibre's ebook viewer

Is there a good alternative one for Mac? The built in one severely cripples copy and paste. Makes copying recipes over into my recipe storage app a massive pain.