diff --git a/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php b/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
index b701344..0a2c6b3 100644
--- a/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
+++ b/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
@@ -143,6 +143,11 @@ public function renderResponse(array $main_content, Request $request, RouteMatch
         ->merge($bubbleable_metadata)
         ->applyTo($html);
     }
+    // Also associate the required cache contexts.
+    // (Because we use ::render() above and not ::renderRoot(), we manually must
+    // ensure the HTML response varies by the required cache contexts.)
+    $html = $this->renderer->applyRequiredCacheContexts($html);
+
     $content = $this->renderCache->getCacheableRenderArray($html);
 
     // Also associate the "rendered" cache tag. This allows us to invalidate the
diff --git a/core/lib/Drupal/Core/Render/Renderer.php b/core/lib/Drupal/Core/Render/Renderer.php
index 7e3d428..4314113 100644
--- a/core/lib/Drupal/Core/Render/Renderer.php
+++ b/core/lib/Drupal/Core/Render/Renderer.php
@@ -255,6 +255,8 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
     // - this is a render cacheable subtree, to ensure that the cached data has
     //   the configurable defaults (which may affect the ID and invalidation).
     if ($is_root_call || isset($elements['#cache']['keys'])) {
+      // This is inlined for performance reasons. Also see
+      // applyRequiredCacheContexts() function below.
       $required_cache_contexts = $this->rendererConfig['required_cache_contexts'];
       if (isset($elements['#cache']['contexts'])) {
         $elements['#cache']['contexts'] = Cache::mergeContexts($elements['#cache']['contexts'], $required_cache_contexts);
@@ -555,6 +557,21 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
   /**
    * {@inheritdoc}
    */
+  public function applyRequiredCacheContexts(array $elements) {
+    $required_cache_contexts = $this->rendererConfig['required_cache_contexts'];
+    if (isset($elements['#cache']['contexts'])) {
+      $elements['#cache']['contexts'] = Cache::mergeContexts($elements['#cache']['contexts'], $required_cache_contexts);
+    }
+    else {
+      $elements['#cache']['contexts'] = $required_cache_contexts;
+    }
+
+    return $elements;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function hasRenderContext() {
     return (bool) $this->getCurrentRenderContext();
   }
diff --git a/core/lib/Drupal/Core/Render/RendererInterface.php b/core/lib/Drupal/Core/Render/RendererInterface.php
index 155eec4..c2c3012 100644
--- a/core/lib/Drupal/Core/Render/RendererInterface.php
+++ b/core/lib/Drupal/Core/Render/RendererInterface.php
@@ -365,6 +365,21 @@ public function hasRenderContext();
   public function executeInRenderContext(RenderContext $context, callable $callable);
 
   /**
+   * Applies the required cache contexts to a render array.
+   *
+   * Note: This function should only be needed in edge-case circumstances; e.g.
+   * when interacting directly with the RenderCache class or by rendering
+   * without replacing placeholders.
+   *
+   * @param array $elements
+   *   The render array to apply the required cache contexts to.
+   *
+   * @return array
+   *   A render array with the required cache contexts added.
+   */
+  public function applyRequiredCacheContexts(array $elements);
+
+  /**
    * Merges the bubbleable rendering metadata o/t 2nd render array with the 1st.
    *
    * @param array $a