Provides tags of all parent HTML elements.

def parent_tags(self):
        """Provides tags of all parent HTML elements."""
        tags = set()

        for addr in self._addresses:
            if addr.attr == 'text':
                tags.add(addr.element.tag)
            tags.update(el.tag for el in addr.element.iterancestors())

        tags.discard(HTMLFragment._root_tag)
        return frozenset(tags)