core/lib/Drupal/Core/Render/MainContent/AjaxRenderer.php   | 2 +-
 core/lib/Drupal/Core/Render/MainContent/DialogRenderer.php | 2 +-
 core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php   | 5 ++---
 3 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/core/lib/Drupal/Core/Render/MainContent/AjaxRenderer.php b/core/lib/Drupal/Core/Render/MainContent/AjaxRenderer.php
index 728f615..766dc2d 100644
--- a/core/lib/Drupal/Core/Render/MainContent/AjaxRenderer.php
+++ b/core/lib/Drupal/Core/Render/MainContent/AjaxRenderer.php
@@ -18,7 +18,7 @@
 /**
  * Default main content renderer for Ajax requests.
  */
-class AjaxRenderer {
+class AjaxRenderer implements MainContentRendererInterface {
 
   /**
    * The controller resolver.
diff --git a/core/lib/Drupal/Core/Render/MainContent/DialogRenderer.php b/core/lib/Drupal/Core/Render/MainContent/DialogRenderer.php
index adb550a..9f59944 100644
--- a/core/lib/Drupal/Core/Render/MainContent/DialogRenderer.php
+++ b/core/lib/Drupal/Core/Render/MainContent/DialogRenderer.php
@@ -16,7 +16,7 @@
 /**
  * Default main content renderer for dialog requests.
  */
-class DialogRenderer {
+class DialogRenderer implements MainContentRendererInterface {
 
   /**
    * The title resolver.
diff --git a/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php b/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
index 4ef398d..739263a 100644
--- a/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
+++ b/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
@@ -9,7 +9,6 @@
 
 use Drupal\Component\Plugin\PluginManagerInterface;
 use Drupal\Component\Utility\NestedArray;
-use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Controller\TitleResolverInterface;
 use Drupal\Core\Display\PageVariantInterface;
@@ -25,7 +24,7 @@
 /**
  * Default main content renderer for HTML requests.
  */
-class HtmlRenderer {
+class HtmlRenderer implements MainContentRendererInterface {
 
   /**
    * The title resolver.
@@ -99,7 +98,7 @@ public function renderResponse(array $main_content, Request $request, RouteMatch
 
     // The special page regions will appear directly in html.html.twig, not in
     // page.html.twig, hence add them here, just before rendering html.html.twig.
-    static::buildPageTopAndBottom($html);
+    $this->buildPageTopAndBottom($html);
 
     // The three parts of rendered markup in html.html.twig (page_top, page and
     // page_bottom) must be rendered with drupal_render_root(), so that their