diff --git a/core/includes/common.inc b/core/includes/common.inc
index c9fdd6f..8444564 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -1098,6 +1098,9 @@ function drupal_render_children(&$element, $children_keys = NULL) {
       $output .= drupal_render($element[$key]);
     }
   }
+  // Output from \Drupal\Core\Render\RendererInterface::render() is always
+  // considered safe. $output is the concatenation of multiple render calls so
+  // therefore is safe as well.
   return SafeMarkup::set($output);
 }