diff --git a/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
index 6850fcb..b562521 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -315,6 +315,8 @@ protected function theme($hook, $variables = array()) {
     $output = '';
     if (isset($info['function'])) {
       if (function_exists($info['function'])) {
+        // Theme functions do not go through Twig so the output is not autoescaped.
+        // Theme function output is always expected to be 100% safe.
         $output = SafeMarkup::set($info['function']($variables));
       }
     }