, attributeOldValue: true});
  function assert_record_with_old_value(oldValue, action) {
    let records = observer.takeRecords();
    assert_equals(records.length, 1, "number of mutation records after " + action);
    let record = records[0];
    assert_equals(record.type, "attributes", "mutation type after " + action);
    assert_equals(record.attributeName, "style", "mutated attribute after " + action);
    assert_equals(record.oldValue, oldValue, "old value after " + action);
  }
  style.setProperty("z-