there are plugins for that (same as you have with PDFs, though you reuse same rwndering engine, since EPUBs are html documents under the hood)
It's far different than being able to link to a PDF file and having it instantly display in the browser. As far as I'm aware, Firefox, Chrome and Safari (and possibly other browsers) can all display PDFs natively, so I think it would be convenient if they could also display EPUBs without the need for additional plugins.
As you say, EPUBs use HTML under the bonnet, so rendering should be pretty straightforward.