diff --git a/core/lib/Drupal/Core/Controller/FormController.php b/core/lib/Drupal/Core/Controller/FormController.php
index 2b0c38b..dc191c0 100644
--- a/core/lib/Drupal/Core/Controller/FormController.php
+++ b/core/lib/Drupal/Core/Controller/FormController.php
@@ -57,8 +57,8 @@ public function __construct(ControllerResolverInterface $controller_resolver, Fo
    *   The render array that results from invoking the controller.
    */
   public function getContentResult(Request $request) {
-    $form_definition = $this->getFormDefinition($request);
-    $form_object = $this->getFormObject($request, $form_definition);
+    $form_arg = $this->getFormArgument($request);
+    $form_object = $this->getFormObject($request, $form_arg);
 
     // Add the form and form_state to trick the getArguments method of the
     // controller resolver.
@@ -77,17 +77,24 @@ public function getContentResult(Request $request) {
   }
 
   /**
-   * Extracts the form definition string from a request.
+   * Extracts the form argument string from a request.
    *
-   * Depending on the type of form the definition string may be stored in a
+   * Depending on the type of form the argument string may be stored in a
    * different request attribute.
    *
+   * One example of a route definition is given below.
+   * @code
+   *   defaults:
+   *     _form: Drupal\example\Form\ExampleForm
+   * @endcode
+   *
    * @param \Symfony\Component\HttpFoundation\Request $request
    *   The request object from which to extract a form definition string.
+   *
    * @return string
    *   The form definition string.
    */
-  abstract protected function getFormDefinition(Request $request);
+  abstract protected function getFormArgument(Request $request);
 
   /**
    * Returns the object used to build the form.
diff --git a/core/lib/Drupal/Core/Controller/HtmlFormController.php b/core/lib/Drupal/Core/Controller/HtmlFormController.php
index ecfa6a1..81a9b02 100644
--- a/core/lib/Drupal/Core/Controller/HtmlFormController.php
+++ b/core/lib/Drupal/Core/Controller/HtmlFormController.php
@@ -9,9 +9,7 @@
 
 use Drupal\Core\Form\FormBuilderInterface;
 use Symfony\Component\HttpFoundation\Request;
-use Symfony\Component\DependencyInjection\ContainerInterface;
 use Drupal\Core\DependencyInjection\ClassResolverInterface;
-use Drupal\Core\Controller\ControllerResolverInterface;
 
 /**
  * Wrapping controller for forms that serve as the main page body.
@@ -19,13 +17,6 @@
 class HtmlFormController extends FormController {
 
   /**
-   * The name of a class implementing FormInterface that defines a form.
-   *
-   * @var string
-   */
-  protected $formClass;
-
-  /**
    * The class resolver.
    *
    * @var \Drupal\Core\DependencyInjection\ClassResolverInterface;
@@ -34,6 +25,13 @@ class HtmlFormController extends FormController {
 
   /**
    * Constructs a new \Drupal\Core\Routing\Enhancer\FormEnhancer object.
+   *
+   * @param \Drupal\Core\Controller\ControllerResolverInterface $controller_resolver
+   *   The controller resolver.
+   * @param \Drupal\Core\Form\FormBuilderInterface $form_builder
+   *   The form builder.
+   * @param \Drupal\Core\DependencyInjection\ClassResolverInterface $class_resolver
+   *   The class resolver.
    */
   public function __construct(ControllerResolverInterface $controller_resolver, FormBuilderInterface $form_builder, ClassResolverInterface $class_resolver) {
     parent::__construct($controller_resolver, $form_builder);
@@ -43,7 +41,7 @@ public function __construct(ControllerResolverInterface $controller_resolver, Fo
   /**
    * @{inheritDoc}
    */
-  protected function getFormDefinition(Request $request) {
+  protected function getFormArgument(Request $request) {
     return $request->attributes->get('_form');
   }
 
diff --git a/core/lib/Drupal/Core/Entity/HtmlEntityFormController.php b/core/lib/Drupal/Core/Entity/HtmlEntityFormController.php
index 971efe0..bd659a9 100644
--- a/core/lib/Drupal/Core/Entity/HtmlEntityFormController.php
+++ b/core/lib/Drupal/Core/Entity/HtmlEntityFormController.php
@@ -42,7 +42,7 @@ public function __construct(ControllerResolverInterface $resolver, FormBuilderIn
   /**
    * @{inheritDoc}
    */
-  protected function getFormDefinition(Request $request) {
+  protected function getFormArgument(Request $request) {
     return $request->attributes->get('_entity_form');
   }
 
diff --git a/core/lib/Drupal/Core/EventSubscriber/ContentControllerSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/ContentControllerSubscriber.php
index faf471a..32f9565 100644
--- a/core/lib/Drupal/Core/EventSubscriber/ContentControllerSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/ContentControllerSubscriber.php
@@ -54,7 +54,7 @@ public function onRequestDeriveFormat(GetResponseEvent $event) {
   }
 
   /**
-   * Sets the _controller on a request based on the request format.
+   * Sets the _controller on a request when a _form is defined.
    *
    * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
    *   The event to process.
@@ -62,8 +62,8 @@ public function onRequestDeriveFormat(GetResponseEvent $event) {
   public function onRequestDeriveFormWrapper(GetResponseEvent $event) {
     $request = $event->getRequest();
 
-    if ($form = $request->attributes->get('_form')) {
-      $request->attributes->set('_content', 'controller.form:getContentResult');
+    if ($request->attributes->has('_form')) {
+      $request->attributes->set('_controller', 'controller.form:getContentResult');
     }
   }