private function css($file, $path = 'css', array $options = [])
    {
        $attr = array_key_exists('attr', $options) ? $options['attr'] : [];
        $attr = array_merge(['rel' => 'stylesheet'], $attr);
        $location = $this->location("{$path}/{$file}", $options);
        return sprintf(
            '<link href="%s" %s>',
            $location,
            $this->attributesStr($attr)
        );
    }