public function html($html_string=null)
    {
        if ($html_string !== null) { // set html for all nodes
            foreach ($this as $node) {
                $node->get(0)->nodeValue = '';
                $node->append($html_string);
            }

            return $this;
        }
        // get html for first node
        return $this->getInnerHtml();
    }