Spec (https://drafts.csswg.org/css-contain/#containment-style):
  "2. The effects of the content property’s
   open-quote, close-quote, no-open-quote and no-close-quote
   must be scoped to the element’s sub-tree."

Tests:
* http://w3c-test.org/css/css-contain/quote-scoping-001.html
* http://w3c-test.org/css/css-contain/quote-scoping-002.html
* http://w3c-test.org/css/css-contain/quote-scoping-003.html
* http://w3c-test.org/css/css-contain/quote-scoping-004.html