Summary:
This brings our implementation into line with the following spec change:

  https://github.com/w3c/csswg-drafts/commit/ced6b5aac02e0e021001f10e4d63e03456686dad

See https://github.com/w3c/csswg-drafts/issues/3000

Reviewers: smaug

Tags: #secure-revision

Bug #: 1483404

Differential Revision: https://phabricator.services.mozilla.com/D3384

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1483404
gecko-commit: a972e755df1b9809ea45aaab942dffea06105900
gecko-integration-branch: mozilla-inbound
gecko-reviewers: smaug