Parse given string as HTML and return it's etree representation.

def _parse(self, html):
        """Parse given string as HTML and return it's etree representation."""
        if self._has_body_re.search(html):
            tree = lxml.html.document_fromstring(html).find('.//body')
            self.has_body = True
        else:
            tree = lxml.html.fragment_fromstring(html,
                                                 create_parent=self._root_tag)

        if tree.tag != self._root_tag:
            # ensure the root element exists even if not really needed,
            # so the tree has always the same structure
            root = lxml.html.HtmlElement()
            root.tag = self._root_tag
            root.append(tree)
            return root
        return tree