diff -u b/core/modules/ckeditor/tests/src/FunctionalJavascript/CKEditorIntegrationTest.php b/core/modules/ckeditor/tests/src/FunctionalJavascript/CKEditorIntegrationTest.php
--- b/core/modules/ckeditor/tests/src/FunctionalJavascript/CKEditorIntegrationTest.php
+++ b/core/modules/ckeditor/tests/src/FunctionalJavascript/CKEditorIntegrationTest.php
@@ -249,13 +249,13 @@
     // So to check if the error has been fixed we toggle the source button and
     // compare the first height to the last height.
     $this->pressEditorButton('source');
-    $this->assertNotEmpty($assert_session->waitForElement('css', '.cke__contents > textarea'));
-    $first_height = $this->getSession()->evaluateScript('document.getElementById(cke_edit-body-0-value).clientHeight');
+    $this->assertNotEmpty($assert_session->waitForElement('css', '.cke_contents > textarea'));
+    $first_height = $this->getSession()->evaluateScript("document.getElementById('cke_edit-body-0-value').clientHeight");
     $this->pressEditorButton('source');
-    $this->assertNotEmpty($assert_session->waitForElement('css', '.cke__contents > span'));
+    $this->assertNotEmpty($assert_session->waitForElement('css', '.cke_contents > span'));
     $this->pressEditorButton('source');
-    $this->assertNotEmpty($assert_session->waitForElement('css', '.cke__contents > textarea'));
-    $last_height = $this->getSession()->evaluateScript('document.getElementById(cke_edit-body-0-value).clientHeight');
+    $this->assertNotEmpty($assert_session->waitForElement('css', '.cke_contents > textarea'));
+    $last_height = $this->getSession()->evaluateScript("document.getElementById('cke_edit-body-0-value').clientHeight");
 
     // If the height is different then empty tag has been entered into the DOM.
     $this->assertEquals($first_height, $last_height);