d be canonical');

      sheet.deleteRule(0);
      assert_equals(cssRules.length, 0, 'Sheet should have no rule');
      sheet.insertRule(serialization);
      assert_equals(cssRules.length, 1, 'Sheet should have 1 rule');

      assert_equals(cssRules[0].cssText, serialization, 'serialization should round-trip');
    }, rule + ' should be a valid supports() import rule');
  }

  function test_unsupported_supports_import(rule) {
    test(function() {
      const {sheet, cssRules} = setupSheet(rule);

     