Ben Mullins
--- a/core/modules/ckeditor5/tests/src/FunctionalJavascript/MediaLibraryTest.php
+++ b/core/modules/ckeditor5/tests/src/FunctionalJavascript/MediaLibraryTest.php
  /**
   * {@inheritdoc}
   */
-  protected $defaultTheme = 'classy';
+  protected $defaultTheme = 'media_library_templates';
--- a/core/modules/ckeditor5/tests/themes/media_library_templates/templates/media-embed-error.html.twig 0 → 100644
+++ b/core/modules/ckeditor5/tests/themes/media_library_templates/templates/media-embed-error.html.twig 0 → 100644
+ * @file
+ * Theme override for a missing media error.
+ *
+ * Available variables
+ * - message: The message text.
+ * - attributes: HTML attributes for the containing element.
+ *
+ * When a response from the back end can't be returned, a related error message
+ * is displayed from JavaScript.
+ *
+ * @see Drupal.theme.mediaEmbedPreviewError
+ *
+ * @ingroup themeable
+ */
+#}
+{{ attach_library('classy/media_embed_error') }}