public function outputCss($container = 'default')
    {
        $assets = $this->getAssets($container, 'css');

        $html = [];

        foreach ($assets as $asset) {
            $source = $this->url ? $this->url->asset($asset['source']): $asset['source'];
            $html[] = $this->html->style($source, $asset['attributes']);
        }

        return trim(implode(PHP_EOL, $html));
    }