There are plugins, but having to find and install one is an extra step that I think a lot of people don't take.
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.
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.