Silence CSS errors in the user-agent stylesheet.

Alternatively, we could comment the parts of that stylesheet that we don’t
support yet. (But we’d have to remember to uncomment them when we do!)
auto merge of #1192 : SimonSapin/servo/silence-ua-stylesheet-errors, r=jdm

Alternatively, we could comment the parts of that stylesheet that we don’t support yet. (But we’d have to remember to uncomment them when we do!)