public function add_rule( $selector ) {
	$selector = trim( $selector );

	// Bail early if there is no selector.
	if ( empty( $selector ) ) {
		return;
	}

	// Create the rule if it doesn't exist.
	if ( empty( $this->rules[ $selector ] ) ) {
		$this->rules[ $selector ] = new WP_Style_Engine_CSS_Rule( $selector );
	}

	return $this->rules[ $selector ];
}