diff --git a/core/lib/Drupal/Component/Utility/Html.php b/core/lib/Drupal/Component/Utility/Html.php
index 00e9016..b735fd3 100644
--- a/core/lib/Drupal/Component/Utility/Html.php
+++ b/core/lib/Drupal/Component/Utility/Html.php
@@ -85,11 +85,12 @@ 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
     // placeholder after checking that it is not defined as a filter.
+    $double_underscore_replacements = 0;
     if (!isset($filter['__'])) {
-      $identifier = str_replace('__', '##', $identifier);
+      $identifier = str_replace('__', '##', $identifier, $double_underscore_replacements);
     }
     $identifier = str_replace(array_keys($filter), array_values($filter), $identifier);
-    if (!isset($filter['__'])) {
+    if (!isset($filter['__']) && $double_underscore_replacements > 0) {
       $identifier = str_replace('##', '__', $identifier);
     }