public function addCss($file, $path='/css', $attr = [])
    {
        $attr = array_merge(['rel' => 'stylesheet'], $attr);
        $output = [];
        foreach ($attr as $name => $value) {
            $output[] = "{$name}=\"{$value}\"";
        }
        $attr = implode(' ', $output);
        $file = str_replace('//', '', "{$path}/{$file}");
        return sprintf('<link href="%s" %s>', $file, $attr);
    }