How do I add the noscript tag?
You can do it in a custom module (MYMODULE.module):

use Drupal\Core\Render\Markup;

function MYMODULE_page_attachments(array &amp;$page) {
  $fallback = '&lt;link href="fallback.css" rel="stylesheet" /&gt;';
  $noscript = array(
    '#tag' =&gt; 'noscript',
    '#value' =&gt; Markup::create($fallback),
  );
  $page['#attached']['html_head'][] = [$noscript, 'mymodule'];
}