public function convert($html) {
    if (preg_match('/^\s*$/u', $html)) {
      return '';
    }
    $html = '<html><head><meta http-equiv="content-type" content="text/html; charset=UTF-8"></head>' . $html . '</body></html>';
    $this->dom = new \DOMDocument();
    if (!$this->dom->loadHTML($html)) {
      return NULL;
    }
    $xp = new \DOMXPath($this->dom);
    return implode(self::BLOCK_DELIMITER, $this->getBlocks(
      $xp->query('/html/body')->item(0)->childNodes
    ));
  }