':param attr: An attribute key/value
:return: True if the attribute value is a valid link'

def _filter_ref(self, attr):

    key = attr[0]
    value = attr[1]
    return (value and (key in self.URL_ATTRS) and (not value.startswith('#')) and (not value.startswith('tel:')) and (not value.startswith('callto:')) and (not (value in self.APACHE_INDEXING)))
