diff --git a/core/lib/Drupal/Component/Utility/SafeMarkup.php b/core/lib/Drupal/Component/Utility/SafeMarkup.php
index 60b3b35..090a73c 100644
--- a/core/lib/Drupal/Component/Utility/SafeMarkup.php
+++ b/core/lib/Drupal/Component/Utility/SafeMarkup.php
@@ -74,6 +74,11 @@ class SafeMarkup {
    *   The input string that was marked as safe.
    */
   public static function set($string, $strategy = 'html') {
+    // Start or increment a counter when a string is marked as safe. This allows
+    // strings to be removed from the array just before printing with
+    // \Drupal\Component\Utility\SafeMarkup::remove() in order to conserve
+    // memory, while accounting for identical strings used within the same
+    // request.
     $string = (string) $string;
     if (!isset(static::$safeStrings[$string][$strategy])) {
       static::$safeStrings[$string][$strategy] = 1;
@@ -99,6 +104,8 @@ public static function set($string, $strategy = 'html') {
    *   - 'all': The string is safe for all use cases.
    */
   public static function remove($string, $strategy = 'html') {
+    // Remove the string if it's only been marked as safe once, otherwise
+    // decrement the counter.
     $string = (string) $string;
     if (isset(static::$safeStrings[$string][$strategy])) {
       if (static::$safeStrings[$string][$strategy] == 1) {
diff --git a/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php b/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
index 5e5bf00..5e03f9b 100644
--- a/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
+++ b/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
@@ -263,7 +263,9 @@ protected function prepare(array $main_content, Request $request, RouteMatchInte
     $this->invokePageAttachmentHooks($page);
 
     // Determine the title: use the title provided by the main content if any,
-    // otherwise get it from the routing information.
+    // otherwise get it from the routing information. The title provided by the
+    // main content was escaped when rendered above. Explicitly mark it as safe
+    // here when re-using the string to avoid double-escaping.
     $title = isset($main_content['#title']) ? SafeMarkup::set($main_content['#title']) : $this->titleResolver->getTitle($request, $route_match->getRouteObject());
 
     return [$page, $title];