Many of our existing reftests and crashtests should be able to run with vertical writing mode set on the root document element; we should do this, to flush out cases where vertical mode causes unexpected breakage.
Jonathan, any progress here? Are these tests failing on mozilla-aurora or are they not enabled?
After some consideration/experiments, I don't think this is something we're going to try and do in automation; we're creating new tests as we go to cover the new functionality. Applying vertical mode "en masse" to existing tests, as suggested here, is more useful as a local tool to flush out issues that can then be isolated and filed as specific bugs.
Let's just WONTFIX this bug, then, per comment 2. (This is the only open dependency of meta bug 1099032 ("enable CSS writing-mode support in developer channels") -- which has already shipped, so it's probably not open to keep this as an open dependency. :))