diff --git a/core/lib/Drupal/Component/Utility/Html.php b/core/lib/Drupal/Component/Utility/Html.php
index 7ed080f675..46b3816459 100644
--- a/core/lib/Drupal/Component/Utility/Html.php
+++ b/core/lib/Drupal/Component/Utility/Html.php
@@ -95,7 +95,6 @@ public static function getClass($class) {
   public static function cleanCssIdentifier($identifier, array $filter = [
     ' ' => '-',
     '_' => '-',
-    '/' => '-',
     '[' => '-',
     ']' => '',
   ]) {