code: protected function getMessagesContainers($type = '')
      {
          if ('' !== $type) {
              $type .= '_';
          }
  
          return $this->getSession()
              ->getPage()
              ->findAll('css', $selector = $this->getDrupalSelector($type . 'message_selector'));
      }
docs: @param string $type
  Message type: "error", "warning", "success" or nothing.
  
  @return NodeElement[]