Also worth pointing out, EPUBs are (or, at least, can be. I'm not sure how much flexibility is in the specifications) basically just bundled HTML.

There’s a fixed layout version of the ePub standard too, allowing PDF quality if that’s what you’re after.

But can you open an epub in a browser? That's the main point here.

Not only simple browser plugins per the other reply (and a plethora of non-crashing mobile apps, whereas mobile PDF reading apps crash on me all the time) - the ePub format is just a zip file in disguise with plain text (HTML) inside and maybe some images/etc.

In a manner of speaking, ePub as a design has an inherent built-in fallback mechanism to manually obtain the internal content in case of failure - including ability to try and repair a broken zip format (zip -F/-FF) and grep it in place (zipgrep).

Yes, but with the plugin