def is_html_link(cls, tag, attribute):
        '''Return whether the link is likely to be external object.'''
        if tag in cls.TAG_ATTRIBUTES \
           and attribute in cls.TAG_ATTRIBUTES[tag]:
            attr_flags = cls.TAG_ATTRIBUTES[tag][attribute]
            return attr_flags & cls.ATTR_HTML

        return attribute == 'href'
Return whether the link is likely to be external object.