;

    assert_equals(cssRules.length, 0, "Sheet should have no rules");
    sheet.insertRule(rule);
    assert_equals(cssRules.length, 1, "Sheet should have 1 rule");

    return {sheet, cssRules};
  }

  function test_valid_supports_import(rule, serialized) {
    if (serialized === undefined)
        serialized = rule;

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

      const serialization = cssRules[0].cssText;
      assert_equals(serialization, serialized, 'serialization shoul