.../Drupal/Core/Cache/CacheableJsonResponse.php    | 26 ++++++++
 .../system/src/Tests/Common/RenderWebTest.php      | 11 ++++
 .../modules/common_test/common_test.services.yml   |  6 ++
 .../src/Render/MainContent/JsonRenderer.php        | 69 ++++++++++++++++++++++
 4 files changed, 112 insertions(+)

diff --git a/core/lib/Drupal/Core/Cache/CacheableJsonResponse.php b/core/lib/Drupal/Core/Cache/CacheableJsonResponse.php
new file mode 100644
index 0000000..bc7d2a3
--- /dev/null
+++ b/core/lib/Drupal/Core/Cache/CacheableJsonResponse.php
@@ -0,0 +1,26 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Cache\CacheableResponse.
+ */
+
+namespace Drupal\Core\Cache;
+
+use Symfony\Component\HttpFoundation\JsonResponse;
+
+/**
+ * A JsonResponse that contains and can expose cacheability metadata.
+ *
+ * Supports Drupal's caching concepts: cache tags for invalidation and cache
+ * contexts for variations.
+ *
+ * @see \Drupal\Core\Cache\Cache
+ * @see \Drupal\Core\Cache\CacheableMetadata
+ * @see \Drupal\Core\Cache\CacheableResponseTrait
+ */
+class CacheableJsonResponse extends JsonResponse implements CacheableResponseInterface {
+
+  use CacheableResponseTrait;
+
+}
diff --git a/core/modules/system/src/Tests/Common/RenderWebTest.php b/core/modules/system/src/Tests/Common/RenderWebTest.php
index e939769..c3dd78b 100644
--- a/core/modules/system/src/Tests/Common/RenderWebTest.php
+++ b/core/modules/system/src/Tests/Common/RenderWebTest.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\system\Tests\Common;
 
+use Drupal\Core\EventSubscriber\MainContentViewSubscriber;
 use Drupal\Core\Url;
 use Drupal\simpletest\WebTestBase;
 
@@ -25,6 +26,16 @@ class RenderWebTest extends WebTestBase {
   public static $modules = array('common_test');
 
   /**
+   * Asserts the cache context for the wrapper format is always present.
+   */
+  function testWrapperFormatCacheContext() {
+    $this->drupalGet('');
+    $this->assertCacheContext('url.query_args:' . MainContentViewSubscriber::WRAPPER_FORMAT);
+    $this->drupalGet('', ['query' => [MainContentViewSubscriber::WRAPPER_FORMAT =>  'json']]);
+    $this->assertCacheContext('url.query_args:' . MainContentViewSubscriber::WRAPPER_FORMAT);
+  }
+
+  /**
    * Tests rendering form elements without passing through
    * \Drupal::formBuilder()->doBuildForm().
    */
diff --git a/core/modules/system/tests/modules/common_test/common_test.services.yml b/core/modules/system/tests/modules/common_test/common_test.services.yml
new file mode 100644
index 0000000..563632f
--- /dev/null
+++ b/core/modules/system/tests/modules/common_test/common_test.services.yml
@@ -0,0 +1,6 @@
+services:
+  main_content_renderer.json:
+    class: Drupal\common_test\Render\MainContent\JsonRenderer
+    arguments: ['@title_resolver', '@renderer']
+    tags:
+      - { name: render.main_content_renderer, format: json }
diff --git a/core/modules/system/tests/modules/common_test/src/Render/MainContent/JsonRenderer.php b/core/modules/system/tests/modules/common_test/src/Render/MainContent/JsonRenderer.php
new file mode 100644
index 0000000..7ebbfd1
--- /dev/null
+++ b/core/modules/system/tests/modules/common_test/src/Render/MainContent/JsonRenderer.php
@@ -0,0 +1,69 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\common_test\Render\MainContent\JsonRenderer.
+ */
+
+namespace Drupal\common_test\Render\MainContent;
+
+use Drupal\Core\Cache\CacheableJsonResponse;
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Controller\TitleResolverInterface;
+use Drupal\Core\Render\MainContent\MainContentRendererInterface;
+use Drupal\Core\Render\RendererInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Default main content renderer for JSON requests.
+ */
+class JsonRenderer implements MainContentRendererInterface {
+
+  /**
+   * The title resolver.
+   *
+   * @var \Drupal\Core\Controller\TitleResolverInterface
+   */
+  protected $titleResolver;
+
+  /**
+   * The renderer service.
+   *
+   * @var \Drupal\Core\Render\RendererInterface
+   */
+  protected $renderer;
+
+  /**
+   * Constructs a new JsonRenderer.
+   *
+   * @param \Drupal\Core\Controller\TitleResolverInterface $title_resolver
+   *   The title resolver.
+   * @param \Drupal\Core\Render\RendererInterface $renderer
+   *   The renderer service.
+   */
+  public function __construct(TitleResolverInterface $title_resolver, RendererInterface $renderer) {
+    $this->titleResolver = $title_resolver;
+    $this->renderer = $renderer;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function renderResponse(array $main_content, Request $request, RouteMatchInterface $route_match) {
+      $json = [];
+
+      $json['content'] = (string) $this->renderer->renderRoot($main_content);
+      if (!empty($main_content['#title'])) {
+        $json['title'] = (string) $main_content['#title'];
+      }
+      else {
+        $json['title'] = (string) $this->titleResolver->getTitle($request, $route_match->getRouteObject());
+      }
+
+      $response = new CacheableJsonResponse($json, 200);
+      $response->addCacheableDependency(CacheableMetadata::createFromRenderArray($main_content));
+      return $response;
+  }
+
+}