docs: def open( self, **kwargs ):
          """Append an opening tag."""
  
          if self.tag in self.parent.twotags or self.tag in self.parent.onetags:
              self.render( self.tag, False, None, kwargs )
          elif self.mode == 'strict_html' and self.tag in self.parent.deptags:
              raise DeprecationError( self.tag )