def _fetch_url(url, is_threaded, timeout=None):
        """
        Crawls the html content of the parameter url and saves the html in _results
        :param url:
        :param is_threaded: If True, results will be stored for later processing by the fetch_urls method. Else not.
        :param timeout: in seconds, if None, the urllib default is used
        :return: html of the url
        """
        headers = {'User-Agent': 'Mozilla/5.0'}
        req = urllib.request.Request(url, None, headers)
        html = urllib.request.urlopen(req, data=None, timeout=timeout).read()

        if is_threaded:
            SimpleCrawler._results[url] = html

        return html