diff --git a/core/lib/Drupal/Component/Utility/Html.php b/core/lib/Drupal/Component/Utility/Html.php
index 0c811f2..a8d8055 100644
--- a/core/lib/Drupal/Component/Utility/Html.php
+++ b/core/lib/Drupal/Component/Utility/Html.php
@@ -78,9 +78,10 @@ public static function getClass($class) {
   public static function cleanCssIdentifier($identifier, array $filter = array(
     ' ' => '-',
     '_' => '-',
+    '__' => '__',
     '/' => '-',
     '[' => '-',
-    ']' => '',
+    ']' => ''
   )) {
     // We could also use strtr() here but its much slower than str_replace(). In
     // order to keep '__' to stay '__' we first replace it with a different