reverted:
--- b/core/modules/ckeditor/tests/modules/ckeditor_test.module
+++ a/core/modules/ckeditor/tests/modules/ckeditor_test.module
@@ -1,2 +1,15 @@
 <?php
 
+/**
+ * @file
+ * Helper module for the CKEditor tests.
+ */
+
+use Drupal\editor\Entity\Editor;
+
+/**
+ * Implements hook_ckeditor_css_alter().
+ */
+function ckeditor_test_ckeditor_css_alter(array &$css, Editor $editor) {
+  $css[] = drupal_get_path('module', 'ckeditor_test') . '/ckeditor_test.css';
+}
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
@@ -44,52 +44,27 @@
     $this->drupalGet('/ckeditor_test/ajax_css');
 
     $session = $this->getSession();
-    $assert = $this->assertSession();
     $page = $session->getPage();
 
-    $style_color = 'rgb(255, 0, 0)';
-
-    $assert->assertWaitOnAjaxRequest();
-
-    // Waiting for CKEDITOR initialization.
-    $condition = <<<JS
-      (function() {
-        return (
-          typeof CKEDITOR !== 'undefined'
-          && typeof CKEDITOR.instances["edit-inline"] !== 'undefined'
-          && CKEDITOR.instances["edit-inline"].instanceReady   
-          && typeof CKEDITOR !== 'undefined'
-          && typeof CKEDITOR.instances["edit-iframe-value"] !== 'undefined'
-          && CKEDITOR.instances["edit-iframe-value"].instanceReady
-        );
-      }());
-JS;
+    $this->waitOnCkeditorInstance('edit-iframe-value');
+    $this->waitOnCkeditorInstance('edit-inline');
 
-    $session->wait('10', $condition);
+    $style_color = 'rgb(255, 0, 0)';
 
     // Add the inline CSS and assert that the style is applied to the main body,
     // but not the iframe.
     $page->pressButton('Add CSS to inline CKEditor instance');
 
-
-
-    $js = sprintf(
-      'CKEDITOR.instances["%s"].document.getBody().getComputedStyle("%s")',
-      'edit-iframe-value',
-      'color'
-    );
-    $session->wait(10, $js);
-
-    $js = sprintf(
-      'CKEDITOR.instances["%s"].document.getBody().getComputedStyle("%s")',
-      'edit-inline',
-      'color'
-    );
-    $session->wait(10, $js);
-
+    $result = $page->waitFor(10, function() use ($style_color) {
+      return ($this->getEditorStyle('edit-inline', 'color') == $style_color)
+        && ($this->getEditorStyle('edit-iframe-value', 'color') != $style_color);
+    });
+    $this->assertTrue($result);
 
     $session->reload();
-    $session->wait('10', $condition);
+
+    $this->waitOnCkeditorInstance('edit-iframe-value');
+    $this->waitOnCkeditorInstance('edit-inline');
 
     // Add the iframe CSS and assert that the style is applied to the iframe,
     // but not the main body.
@@ -125,2 +100,23 @@
 
+  /**
+   * Wait for a CKEditor instance to finish loading and initializing.
+   *
+   * @param string $instance_id
+   *   The CKEditor instance ID.
+   * @param int $timeout
+   *   (optional) Timeout in milliseconds, defaults to 10000.
+   */
+  protected function waitOnCkeditorInstance($instance_id, $timeout = 10000) {
+    $condition = <<<JS
+      (function() {
+        return (
+          typeof CKEDITOR !== 'undefined'
+          && typeof CKEDITOR.instances["$instance_id"] !== 'undefined'
+          && CKEDITOR.instances["$instance_id"].instanceReady
+        );
+      }());
+JS;
+
+    $this->getSession()->wait($timeout, $condition);
+  }
 }
only in patch2:
unchanged:
--- a/core/scripts/run-tests.sh
+++ b/core/scripts/run-tests.sh
@@ -142,6 +142,16 @@
 
 $test_list = simpletest_script_get_test_list();
 
+if (in_array('Drupal\Tests\ckeditor\FunctionalJavascript\AjaxCssTest', $test_list)) {
+  // Do the test 25 times.
+  $test_list = array_fill(0, 25, 'Drupal\Tests\ckeditor\FunctionalJavascript\AjaxCssTest');
+}
+else {
+  $test_list = [];
+}
+
+
+
 // Try to allocate unlimited time to run the tests.
 drupal_set_time_limit(0);
 simpletest_script_reporter_init();