diff --git a/core/lib/Drupal/Component/Utility/Html.php b/core/lib/Drupal/Component/Utility/Html.php
index d4bf13e..fb5234e 100644
--- a/core/lib/Drupal/Component/Utility/Html.php
+++ b/core/lib/Drupal/Component/Utility/Html.php
@@ -78,12 +78,17 @@ public static function getClass($class) {
   public static function cleanCssIdentifier($identifier, array $filter = array(
     ' ' => '-',
     '_' => '-',
-    '__' => '__',
     '/' => '-',
     '[' => '-',
     ']' => ''
   )) {
-    $identifier = strtr($identifier, $filter);
+    // 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.
+    $identifier = str_replace('__', '##', $identifier);
+    $identifier = str_replace(array_keys($filter), array_values($filter), $identifier);
+    $identifier = str_replace('##', '__', $identifier);
+
     // Valid characters in a CSS identifier are:
     // - the hyphen (U+002D)
     // - a-z (U+0030 - U+0039)