function saveClass($className = '') {
        $css = $this->css();
        if ($css === '') return false;
        if (isset(self::$_cssIndex[$css])) {
            $className = self::$_cssIndex[$css];
        } else {
            if ($className === '') {
                $classNamePrefix = 'class';
                $className = $classNamePrefix . rand(0, 2000000000);
            } else {
                $classNamePrefix = $className;
            }
            while (isset(self::$_classes[$className])) {
                $className = $classNamePrefix . rand(0, 2000000000);
            }
            self::$_classes[$className] = true;
            self::$_cssIndex[$css] = $className;
            $this->_assets->addStyle($css, '.'.$className);
        }

        return $className;
    }