diff -u b/core/modules/ckeditor/tests/modules/src/Kernel/CKEditorTest.php b/core/modules/ckeditor/tests/modules/src/Kernel/CKEditorTest.php
--- b/core/modules/ckeditor/tests/modules/src/Kernel/CKEditorTest.php
+++ b/core/modules/ckeditor/tests/modules/src/Kernel/CKEditorTest.php
@@ -346,6 +346,7 @@
     $editor->save();
     $this->assertIdentical($expected, $stylescombo_plugin->getConfig($editor), '"StylesCombo" plugin configuration built correctly for customized toolbar.');
 
+    // Configuration that includes a dash in the class name.
     $settings['plugins']['stylescombo']['styles'] = "  p .test-class   |  classwithDash ";
     $editor->setSettings($settings);
     $editor->save();