diff --git a/core/lib/Drupal/Component/Utility/Html.php b/core/lib/Drupal/Component/Utility/Html.php
index cb41775..fb5234e 100644
--- a/core/lib/Drupal/Component/Utility/Html.php
+++ b/core/lib/Drupal/Component/Utility/Html.php
@@ -82,6 +82,9 @@ 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.
     $identifier = str_replace('__', '##', $identifier);
     $identifier = str_replace(array_keys($filter), array_values($filter), $identifier);
     $identifier = str_replace('##', '__', $identifier);