diff --git a/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php b/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
index ffbd8f3..ef8b420 100644
--- a/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
+++ b/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
@@ -7,9 +7,12 @@
 
 namespace Drupal\Core\Form\EventSubscriber;
 
+use Drupal\Core\EventSubscriber\MainContentViewSubscriber;
 use Drupal\Core\Form\FormAjaxException;
 use Drupal\Core\Form\FormAjaxResponseBuilderInterface;
+use Drupal\Core\Form\FormBuilderInterface;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+use Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent;
 use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
 use Symfony\Component\HttpKernel\KernelEvents;
 
@@ -36,6 +39,21 @@ public function __construct(FormAjaxResponseBuilderInterface $form_ajax_response
   }
 
   /**
+   * Alters the wrapper format if this is an AJAX form request.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent $event
+   *   The event to process.
+   */
+  public function onView(GetResponseForControllerResultEvent $event) {
+    // Ensure this request is rendered as HTML so that we can manage the
+    // creation of an AJAX response directly in self::onException().
+    $request = $event->getRequest();
+    if ($request->query->has(FormBuilderInterface::AJAX_FORM_REQUEST)) {
+      $request->query->set(MainContentViewSubscriber::WRAPPER_FORMAT, 'html');
+    }
+  }
+
+  /**
    * Catches a form AJAX exception and build a response from it.
    *
    * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
@@ -95,6 +113,8 @@ protected function getFormAjaxException(\Exception $e) {
   public static function getSubscribedEvents() {
     // Run before exception.logger.
     $events[KernelEvents::EXCEPTION] = ['onException', 51];
+    // Run before main_content_view_subscriber.
+    $events[KernelEvents::VIEW][] = ['onView', 1];
 
     return $events;
   }
diff --git a/core/lib/Drupal/Core/Form/FormAjaxResponseBuilder.php b/core/lib/Drupal/Core/Form/FormAjaxResponseBuilder.php
index 4b826ff..293475c 100644
--- a/core/lib/Drupal/Core/Form/FormAjaxResponseBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormAjaxResponseBuilder.php
@@ -60,8 +60,7 @@ public function buildResponse(Request $request, array $form, FormStateInterface
 
     // We need to return the part of the form (or some other content) that needs
     // to be re-rendered so the browser can update the page with changed
-    // content. Since this is the generic menu callback used by many Ajax
-    // elements, it is up to the #ajax['callback'] function of the element (may
+    // content. It is up to the #ajax['callback'] function of the element (may
     // or may not be a button) that triggered the Ajax request to determine what
     // needs to be rendered.
     $callback = NULL;
diff --git a/core/lib/Drupal/Core/Form/FormBuilderInterface.php b/core/lib/Drupal/Core/Form/FormBuilderInterface.php
index 125199d..60ad704 100644
--- a/core/lib/Drupal/Core/Form/FormBuilderInterface.php
+++ b/core/lib/Drupal/Core/Form/FormBuilderInterface.php
@@ -13,7 +13,17 @@
 interface FormBuilderInterface {
 
   /**
-   * Request key for a form that was triggered via AJAX.
+   * Request key for AJAX forms that submit to the form's original route.
+   *
+   * This constant is distinct from a "drupal_ajax" value for
+   * \Drupal\Core\EventSubscriber\MainContentViewSubscriber::WRAPPER_FORMAT,
+   * because that one is set for all AJAX submissions, including ones with
+   * dedicated routes for which self::buildForm() should not exit early via a
+   * \Drupal\Core\Form\FormAjaxException.
+   *
+   * @todo Re-evaluate the need for this constant after
+   *   https://www.drupal.org/node/2502785 and
+   *   https://www.drupal.org/node/2503429.
    */
   const AJAX_FORM_REQUEST = 'ajax_form';
 
diff --git a/core/lib/Drupal/Core/Render/Element/RenderElement.php b/core/lib/Drupal/Core/Render/Element/RenderElement.php
index 73b4c36..c6f5944 100644
--- a/core/lib/Drupal/Core/Render/Element/RenderElement.php
+++ b/core/lib/Drupal/Core/Render/Element/RenderElement.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\Core\Render\Element;
 
-use Drupal\Core\EventSubscriber\MainContentViewSubscriber;
 use Drupal\Core\Form\FormBuilderInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Plugin\PluginBase;
@@ -129,10 +128,14 @@ public static function preRenderGroup($element) {
    * @see self::preRenderAjaxForm()
    */
   public static function processAjaxForm(&$element, FormStateInterface $form_state, &$complete_form) {
-    if (!empty($element['#ajax']['url'])) {
+    $element = static::preRenderAjaxForm($element);
+
+    // If the element was processed as an #ajax element, and a custom URL was
+    // provided, set the form to be cached.
+    if (!empty($element['#ajax_processed']) && !empty($element['#ajax']['url'])) {
       $form_state->setCached();
     }
-    return static::preRenderAjaxForm($element);
+    return $element;
   }
 
   /**
@@ -237,12 +240,11 @@ public static function preRenderAjaxForm($element) {
       // to be substantially different for a JavaScript triggered submission.
       // One such substantial difference is form elements that use
       // #ajax['callback'] for determining which part of the form needs
-      // re-rendering. For that, we have a special 'system/ajax' route.
-      if (isset($settings['callback']) && !isset($settings['url'])) {
+      // re-rendering. For that, we have a special 'system.ajax' route which
+      // must be manually set.
+      if (array_key_exists('callback', $settings) && !isset($settings['url'])) {
         $settings['url'] = Url::fromRoute('<current>');
         $settings['options']['query'][FormBuilderInterface::AJAX_FORM_REQUEST] = TRUE;
-        // Specify that we expect HTML back, despite the AJAX request.
-        $settings['options']['query'][MainContentViewSubscriber::WRAPPER_FORMAT] = 'html';
       }
       $settings += array(
         'url' => NULL,
diff --git a/core/modules/file/src/Controller/FileWidgetAjaxController.php b/core/modules/file/src/Controller/FileWidgetAjaxController.php
index 6cc1391..90176f8 100644
--- a/core/modules/file/src/Controller/FileWidgetAjaxController.php
+++ b/core/modules/file/src/Controller/FileWidgetAjaxController.php
@@ -10,12 +10,7 @@
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Ajax\AjaxResponse;
 use Drupal\Core\Ajax\ReplaceCommand;
-use Drupal\Core\Form\FormAjaxResponseBuilderInterface;
-use Drupal\Core\Form\FormBuilderInterface;
-use Drupal\Core\Render\RendererInterface;
 use Drupal\system\Controller\FormAjaxController;
-use Psr\Log\LoggerInterface;
-use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\HttpFoundation\JsonResponse;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpKernel\Exception\HttpExceptionInterface;
@@ -26,42 +21,6 @@
 class FileWidgetAjaxController extends FormAjaxController {
 
   /**
-   * The renderer.
-   *
-   * @var \Drupal\Core\Render\RendererInterface
-   */
-  protected $renderer;
-
-  /**
-   * Constructs a FileWidgetAjaxController object.
-   *
-   * @param \Psr\Log\LoggerInterface $logger
-   *   A logger instance.
-   * @param \Drupal\Core\Form\FormBuilderInterface $form_builder
-   *   The form builder.
-   * @param \Drupal\Core\Form\FormAjaxResponseBuilderInterface $form_ajax_response_builder
-   *   The form AJAX response builder.
-   * @param \Drupal\Core\Render\RendererInterface $renderer
-   *   The renderer.
-   */
-  public function __construct(LoggerInterface $logger, FormBuilderInterface $form_builder, FormAjaxResponseBuilderInterface $form_ajax_response_builder, RendererInterface $renderer) {
-    parent::__construct($logger, $form_builder, $form_ajax_response_builder);
-    $this->renderer = $renderer;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public static function create(ContainerInterface $container) {
-    return new static(
-      $container->get('logger.factory')->get('ajax'),
-      $container->get('form_builder'),
-      $container->get('form_ajax_response_builder'),
-      $container->get('renderer')
-    );
-  }
-
-  /**
    * Processes AJAX file uploads and deletions.
    *
    * @param \Symfony\Component\HttpFoundation\Request $request
diff --git a/core/modules/system/src/Controller/FormAjaxController.php b/core/modules/system/src/Controller/FormAjaxController.php
index 65078a8..4908197 100644
--- a/core/modules/system/src/Controller/FormAjaxController.php
+++ b/core/modules/system/src/Controller/FormAjaxController.php
@@ -11,6 +11,9 @@
 use Drupal\Core\Form\FormAjaxResponseBuilderInterface;
 use Drupal\Core\Form\FormState;
 use Drupal\Core\Form\FormBuilderInterface;
+use Drupal\Core\Render\MainContent\MainContentRendererInterface;
+use Drupal\Core\Render\RendererInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\system\FileAjaxForm;
 use Psr\Log\LoggerInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
@@ -37,6 +40,27 @@ class FormAjaxController implements ContainerInjectionInterface {
   protected $formBuilder;
 
   /**
+   * The renderer.
+   *
+   * @var \Drupal\Core\Render\RendererInterface
+   */
+  protected $renderer;
+
+  /**
+   * The main content to AJAX Response renderer.
+   *
+   * @var \Drupal\Core\Render\MainContent\MainContentRendererInterface
+   */
+  protected $ajaxRenderer;
+
+  /**
+   * The current route match.
+   *
+   * @var \Drupal\Core\Routing\RouteMatchInterface
+   */
+  protected $routeMatch;
+
+  /**
    * The form AJAX response builder.
    *
    * @var \Drupal\Core\Form\FormAjaxResponseBuilderInterface
@@ -50,12 +74,19 @@ class FormAjaxController implements ContainerInjectionInterface {
    *   A logger instance.
    * @param \Drupal\Core\Form\FormBuilderInterface $form_builder
    *   The form builder.
+   * @param \Drupal\Core\Render\RendererInterface $renderer
+   *   The renderer.
+   * @param \Drupal\Core\Render\MainContent\MainContentRendererInterface $ajax_renderer
+   *   The main content to AJAX Response renderer.
+   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
+   *   The current route match.
    * @param \Drupal\Core\Form\FormAjaxResponseBuilderInterface $form_ajax_response_builder
    *   The form AJAX response builder.
    */
-  public function __construct(LoggerInterface $logger, FormBuilderInterface $form_builder, FormAjaxResponseBuilderInterface $form_ajax_response_builder) {
+  public function __construct(LoggerInterface $logger, FormBuilderInterface $form_builder, RendererInterface $renderer, MainContentRendererInterface $ajax_renderer, RouteMatchInterface $route_match, FormAjaxResponseBuilderInterface $form_ajax_response_builder) {
     $this->logger = $logger;
     $this->formBuilder = $form_builder;
+    $this->renderer = $renderer;
     $this->formAjaxResponseBuilder = $form_ajax_response_builder;
   }
 
@@ -66,6 +97,9 @@ public static function create(ContainerInterface $container) {
     return new static(
       $container->get('logger.factory')->get('ajax'),
       $container->get('form_builder'),
+      $container->get('renderer'),
+      $container->get('main_content_renderer.ajax'),
+      $container->get('current_route_match'),
       $container->get('form_ajax_response_builder')
     );
   }
diff --git a/core/modules/system/src/Tests/Ajax/AjaxFormCacheTest.php b/core/modules/system/src/Tests/Ajax/AjaxFormCacheTest.php
index b70040f..f28adf1 100644
--- a/core/modules/system/src/Tests/Ajax/AjaxFormCacheTest.php
+++ b/core/modules/system/src/Tests/Ajax/AjaxFormCacheTest.php
@@ -17,7 +17,7 @@
 class AjaxFormCacheTest extends AjaxTestBase {
 
   /**
-   * {@inheritdoc}
+   * Tests the usage of form cache for AJAX forms.
    */
   public function testFormCacheUsage() {
     /** @var \Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface $key_value_expirable */
diff --git a/core/modules/system/src/Tests/Ajax/DialogTest.php b/core/modules/system/src/Tests/Ajax/DialogTest.php
index e9e90b7..3422c4e 100644
--- a/core/modules/system/src/Tests/Ajax/DialogTest.php
+++ b/core/modules/system/src/Tests/Ajax/DialogTest.php
@@ -168,7 +168,6 @@ public function testDialog() {
         'event' => 'click',
         'url' => Url::fromRoute('ajax_test.dialog_form', [], ['query' => [
             FormBuilderInterface::AJAX_FORM_REQUEST => TRUE,
-            MainContentViewSubscriber::WRAPPER_FORMAT => 'html',
           ]])->toString(),
         'dialogType' => 'ajax',
         'submit' => [
diff --git a/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestSimpleForm.php b/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestSimpleForm.php
index 1f76850..225664d 100644
--- a/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestSimpleForm.php
+++ b/core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestSimpleForm.php
@@ -10,7 +10,6 @@
 use Drupal\Core\Form\FormBase;
 use Drupal\ajax_forms_test\Callbacks;
 use Drupal\Core\Form\FormStateInterface;
-use Drupal\Core\Url;
 
 /**
  * Form builder: Builds a form that triggers a simple AJAX callback.
@@ -69,11 +68,7 @@ public function buildForm(array $form, FormStateInterface $form_state) {
         '#type' => 'select',
         '#title' => $this->t('Test %key callbacks', array('%key' => $key)),
         '#options' => array('red' => 'red'),
-        '#ajax' => array(
-          'callback' => $value,
-          // If the callback is NULL, the URL needs to be specified.
-          'url' => $value === NULL ? Url::fromRoute('system.ajax') : NULL,
-        ),
+        '#ajax' => array('callback' => $value),
       );
     }