Policy for reverting changes

Hi all,

I think accidental breakage is going to become increasingly common in wpt
as browser vendors use it more and more, and need to work on its
infrastructure more and more. This happened recently. And there was a lack
of perfect consensus on how we should handle CSSWG tooling.

I've filed https://github.com/w3c/web-platform-tests/issues/9953, please
discuss there!