For CSS linting, we also run the code through a gecko parser to check for errors and warnings, and it appears that we're stuck with b2g34 because of some mac issues.
The particular rule that spawned this discussion is "object-fit: cover", and it seems to require at least b2g36[1], so that's another reason to add to xfail.list, until we fix both steps of the linting process. This makes me sad.
[1]
http://caniuse.com/object-fit