public function addCssRequirement($css, LambdaHelper $helper = null)
    {
        $css = trim($css);
        $key = md5($css);

        if (!isset(self::$cssRequirements[$key])) {
            if ($helper !== null) {
                $css = $helper->render($css);
            }

            self::$cssRequirements[$key] = $css;
        }
    }