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