I was just going to ask why this was better than just saving an MHTML file in the browser.
Maybe that's the reason.
https://connect.mozilla.org/t5/ideas/add-support-of-mhtml-fo...
[1] https://docs.google.com/document/d/1FvmYUC0S0BkdkR7wZsg0hLdK...
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.