core/modules/ckeditor/src/Plugin/CKEditorPlugin/Internal.php | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/core/modules/ckeditor/src/Plugin/CKEditorPlugin/Internal.php b/core/modules/ckeditor/src/Plugin/CKEditorPlugin/Internal.php
index c405c7f..9f6586d 100644
--- a/core/modules/ckeditor/src/Plugin/CKEditorPlugin/Internal.php
+++ b/core/modules/ckeditor/src/Plugin/CKEditorPlugin/Internal.php
@@ -127,6 +127,9 @@ public function getConfig(Editor $editor) {
    */
   public function getButtons() {
     $button = function($name, $direction = 'ltr') {
+      // Note: this uses str_replace() instead of Html::cleanCssIdentifier()
+      // because this generates HTML that CKEditor's CSS is targeting, not
+      // Drupal's CSS.
       return '<a href="#" class="cke-icon-only cke_' . $direction . '" role="button" title="' . $name . '" aria-label="' . $name . '"><span class="cke_button_icon cke_button__' . str_replace(' ', '', $name) . '_icon">' . $name . '</span></a>';
     };