framework/web/View.php
public function registerCssFile($url, $options = [], $key = null)
    {
        $url = Yii::getAlias($url);
        $key = $key ?: $url;

        $depends = ArrayHelper::remove($options, 'depends', []);

        if (empty($depends)) {
            $this->cssFiles[$key] = Html::cssFile($url, $options);
        } else {
            $this->getAssetManager()->bundles[$key] = Yii::createObject([
                'class' => AssetBundle::className(),
                'baseUrl' => '',
                'css' => [strncmp($url, '//', 2) === 0 ? $url : ltrim($url, '/')],
                'cssOptions' => $options,
                'depends' => (array) $depends,
            ]);
            $this->registerAssetBundle($key);
        }
    }
public function registerCssFile($url, $options = [], $key = null)
    {
        $url = Yii::getAlias($url);
        $key = $key ?: $url;

        $depends = ArrayHelper::remove($options, 'depends', []);

        if (empty($depends)) {
            $this->cssFiles[$key] = Html::cssFile($url, $options);
        } else {
            $this->getAssetManager()->bundles[$key] = Yii::createObject([
                'class' => AssetBundle::className(),
                'baseUrl' => '',
                'css' => [strncmp($url, '//', 2) === 0 ? $url : ltrim($url, '/')],
                'cssOptions' => $options,
                'depends' => (array) $depends,
            ]);
            $this->registerAssetBundle($key);
        }
    }
Registers a CSS file.

This method should be used for simple registration of CSS files. If you want to use features of
[[AssetManager]] like appending timestamps to the URL and file publishing options, use [[AssetBundle]]
and [[registerAssetBundle()]] instead.

@param string $url the CSS file to be registered.
@param array $options the HTML attributes for the link tag. Please refer to [[Html::cssFile()]] for
the supported options. The following options are specially handled and are not treated as HTML attributes:

- `depends`: array, specifies the names of the asset bundles that this CSS file depends on.

@param string $key the key that identifies the CSS script file. If null, it will use
$url as the key. If two CSS files are registered with the same key, the latter
will overwrite the former.