code: public static function cssFile($url, $options = [])
      {
          if (!isset($options['rel'])) {
              $options['rel'] = 'stylesheet';
          }
          $options['href'] = Url::to($url);
  
          if (isset($options['condition'])) {
              $condition = $options['condition'];
              unset($options['condition']);
              return self::wrapIntoCondition(static::tag('link', '', $options), $condition);
          } elseif (isset($options['noscript']) && $options['noscript'] === true) {
              unset($options['noscript']);
              return '<noscript>' . static::tag('link', '', $options) . '</noscript>';
          }
  
          return static::tag('link', '', $options);
      }
docs: Generates a link tag that refers to an external CSS file.
  @param array|string $url the URL of the external CSS file. This parameter will be processed by [[Url::to()]].
  @param array $options the tag options in terms of name-value pairs. The following options are specially handled:
  
  - condition: specifies the conditional comments for IE, e.g., `lt IE 9`. When this is specified,
  the generated `link` tag will be enclosed within the conditional comments. This is mainly useful
  for supporting old versions of IE browsers.
  - noscript: if set to true, `link` tag will be wrapped into `<noscript>` tags.
  
  The rest of the options will be rendered as the attributes of the resulting link tag. The values will
  be HTML-encoded using [[encode()]]. If a value is null, the corresponding attribute will not be rendered.
  See [[renderTagAttributes()]] for details on how attributes are being rendered.
  @return string the generated link tag
  @see Url::to()