Add or retrieve data values for this :class:`Html`.

def data(self, *args):
        '''Add or retrieve data values for this :class:`Html`.'''
        data = self._data
        if not args:
            return data or {}
        result, adding = self._attrdata('data', *args)
        if adding:
            if data is None:
                self._extra['data'] = {}
            add = self._visitor.add_data
            for key, value in result.items():
                add(self, key, value)
            return self
        else:
            return result