diff --git a/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
index 6850fcb..5cd4b3d 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -312,6 +312,8 @@ protected function theme($hook, $variables = array()) {
     }
 
     // Generate the output using either a function or a template.
+    // Theme functions do not go through Twig so the output is not autoescaped.
+    // Theme function output is always expected to be 100% safe.
     $output = '';
     if (isset($info['function'])) {
       if (function_exists($info['function'])) {