returns WHATWG spec HTML fragment from selector expression.

def select(self, html, stype, expression):
        """returns WHATWG spec HTML fragment from selector expression."""
        etree = html5lib.parse(html,
                               treebuilder='lxml',
                               namespaceHTMLElements=False)
        if stype == 'css':
            selector = lxml.cssselect.CSSSelector(expression)
            frag = list(selector(etree))
        else:
            frag = etree.xpath(expression)
        if not frag:
            raise RuntimeError("Nothing found for: %s" % expression)
        return "".join([lxml.etree.tostring(x) for x in frag])