docs: function getInputValue(inputElement) {
      if (inputElement.type) {
          if (inputElement.type.toUpperCase() == 'CHECKBOX' ||
              inputElement.type.toUpperCase() == 'RADIO')
          {
              return (inputElement.checked ? 'on' : 'off');
          }
      }
      if (inputElement.value == null) {
          throw new SeleniumError("This element has no value; is it really a form field?");
      }
      return inputElement.value;
  }