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I'm actually referring to the "set of modifications" described here [1] and the fact that only Chromium can open (these) MHTML files today.

Good to know that!

Are you meaning that these modifications cause the generated MHTML files to become non-standards-compliant?

I wouldn't consider them proprietary though. At least the primary software used to open these files, Chromium, is open-source software.

I agree that it's debatable, which is why I used the word “almost” ;). The fact remains that this info isn't in RFCs and is hard to find on the web.