No it isn't. There's more to Epub than HTML and web browsers don't know by default how deal with the files or handle things like pagination or the table of contents. Mobi is also based on HTML too. Doesn't mean every web browser supports mobi either.

> and web browsers don't know by default how deal with the files or handle things like pagination or the table of contents.

So? Safari and Chrome don't know by default how to handle pagination of documents. (Actually, they do; they handle it by scrolling.) But any browser that exists on a Kindle does. If you can use it to read a web page, you can also use it to read an epub.

I had to load a Firefox extension[0] to read ePub. I don’t think it can read them without it.

[0] https://addons.mozilla.org/en-US/firefox/addon/epubreader/

Sure; the book is a website, not a webpage. The harness has to do three things:

1. Unzip the file.

2. Display the content root.

3. Handle the implicit transition from one chapter to the next.

All of the work involved here is in step 2, writing the web renderer.