MHTML is a neat format, it's unfortunate it never got much steam. I think it could have been more popular if web browsers had defaulted to it when saving pages, rather than this weird html + _files/ directory (which on Windows is mysteriously linked so that when you delete one, you delete the other - no idea how they do that!).

What I've read of EPUB is also pretty disappointing. Seeing as it's a compiled format, once again, instead of going the zipfile + bunch of html inside + specific layout, we could have had a subset of html in .mhtml.gz with, like, metadata in a <script type="application/json" id="x-epub-metadata">. And then, guess what, web browsers could have been able to read it natively…