framework/helpers/BaseHtml.php
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', '', $options);
    }
public static function cssFile($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);
    }
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()