> And an epub is just a collection of HTML files anyway, so it shouldn't be difficult to make a website instead. I think what's missing is a "website archive format" supported by all major browsers, so the user can have an offline copy of the "book".

Seems to me that, since an ePub is just a collection of HTML files (well, there's more than that -- there's also an index file, at least, IIRC), the solution is that browsers should be enhanced to read ePub directly. You've already got the "website archive format", you just lack the browser support.