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Add cross-origin opener policy This commit adds the notion of cross-origin opener policy (COOP). COOP allows websites to restrict which origins they share their browsing context group with. annevk wrote a first draft of the behavior of COOP here: This takes that draft and merges it into the spec, with many updates along the way. Closes #3740. Closes #4580. Closes #4921. Closes #5172. Co-authored-by: clamy <> Co-authored-by: Anne van Kesteren <> Co-authored-by: Domenic Denicola <>