layout/style/crashtests/645951-1.html fails when stylesheet inner cloning is fixed in Bug 1339629. It may have only been trivially passing before since BOTH stylesheets were getting the new rule. Some cascading error is actually present.

P3 because that's what we mark bugs of the form "fix test X", but feel free to work on it if it makes sense as direct followup investigation to your other work.

So I think we need to clone all the child stylesheets in Gecko side in addition to those in the Servo side. @import rule may depend on this as well, but I'm not going to wait for this, but instead add some warnings there.