I had never heard about MHTML either. Another use case could be embedding markdown source for the HTML in the document as well. This would allow single-file documents with figures that could be edited as light markup (with some tooling) and be viewed by anyone with a browser. This is something I have been dreaming about for years!

Tbh. I had arrived at the conclusion that Mime would be great, but it never struck me that someone had already made a "standard" of mime and HTML.