It appears that the video player on Mac has problems two things essential to the 'object-fit' property:
2: forcing an aspect ratio different from the intrinsic one
Disclaimer: I don't have a Mac, so I haven't really investigated this. Could be that some of this actually needs to be fixed in WebCore.
Two new tests added in bug 52040 fail because of this:
Added to TestExpectations accordingly.
These tests are failing very often, but not in the way described here any more. I'm just seeing them basically time out, rendering some of the boxes black.
Filed bug 121803 to track the current behavior.