diff -u b/core/modules/ckeditor/tests/modules/js/ajax-css.js b/core/modules/ckeditor/tests/modules/js/ajax-css.js
--- b/core/modules/ckeditor/tests/modules/js/ajax-css.js
+++ b/core/modules/ckeditor/tests/modules/js/ajax-css.js
@@ -1,3 +1,8 @@
+/**
+ * @file
+ * Contains client-side code for testing CSS delivered to CKEditor via AJAX.
+ */
+
 (function (Drupal, ckeditor, editorSettings, $) {
 
   'use strict';
diff -u b/core/modules/ckeditor/tests/modules/src/Form/AjaxCssForm.php b/core/modules/ckeditor/tests/modules/src/Form/AjaxCssForm.php
--- b/core/modules/ckeditor/tests/modules/src/Form/AjaxCssForm.php
+++ b/core/modules/ckeditor/tests/modules/src/Form/AjaxCssForm.php
@@ -7,6 +7,9 @@
 use Drupal\Core\Form\FormBase;
 use Drupal\Core\Form\FormStateInterface;
 
+/**
+ * A form for testing delivery of CSS to CKEditor via AJAX.
+ */
 class AjaxCssForm extends FormBase {
 
   /**
diff -u b/core/modules/ckeditor/tests/src/FunctionalJavascript/AjaxCssTest.php b/core/modules/ckeditor/tests/src/FunctionalJavascript/AjaxCssTest.php
--- b/core/modules/ckeditor/tests/src/FunctionalJavascript/AjaxCssTest.php
+++ b/core/modules/ckeditor/tests/src/FunctionalJavascript/AjaxCssTest.php
@@ -7,6 +7,8 @@
 use Drupal\FunctionalJavascriptTests\JavascriptTestBase;
 
 /**
+ * Tests delivery of CSS to CKEditor via AJAX.
+ *
  * @group ckeditor
  */
 class AjaxCssTest extends JavascriptTestBase {