def css(self, *props, **kwprops):
        """Adds css properties to this element."""
        self._stable = False
        styles = {}
        if props:
            if len(props) == 1 and isinstance(props[0], Mapping):
                styles = props[0]
            else:
                raise WrongContentError(self, props, "Arguments not valid")
        elif kwprops:
            styles = kwprops
        else:
            raise WrongContentError(self, None, "args OR wkargs are needed")
        return self.attr(style=styles)
Adds css properties to this element.