private static function mergeCssClasses(array $existingClasses, array $additionalClasses)
    {
        foreach ($additionalClasses as $key => $class) {
            if (is_int($key) && !in_array($class, $existingClasses)) {
                $existingClasses[] = $class;
            } elseif (!isset($existingClasses[$key])) {
                $existingClasses[$key] = $class;
            }
        }

        return array_unique($existingClasses);
    }