There are already some tests in third_party/WebKit/LayoutTests/mhtml (for most of these tests, verification is done by comparing a foo.mhtml file with an foo-expected.html reference file).  OTOH, there might be some extra work needed to make these tests compatible with the "WPT reftests guidelines": https://chromium.googlesource.com/chromium/src/+/master/docs/testing/writing_layout_tests.md#Reference-Tests-Reftests

Is there anyone who can own this bug?

I don't think anything will happen here in the immediate term, let's just have it linked to issue 707649 for tracking purposes, in case it becomes relevant again. Decreasing prio.

This issue has been Available for over a year. If it's no longer important or seems unlikely to be fixed, please consider closing it out. If it is important, please re-triage the issue.

Sorry for the inconvenience if the bug really should have been left as Available.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot