Now that I think about it I'm kind of surprised that browser don't natively show EPUB files.
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.