diff --git a/core/lib/Drupal/Core/EventSubscriber/MainContentViewSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/MainContentViewSubscriber.php
index c9ad6f8..e1fe0af 100644
--- a/core/lib/Drupal/Core/EventSubscriber/MainContentViewSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/MainContentViewSubscriber.php
@@ -80,7 +80,7 @@ public function onViewRenderArray(GetResponseForControllerResultEvent $event) {
 
     // Render the controller result into a response if it's a render array.
     if (is_array($result)) {
-      $wrapper = $request->query->get(static::WRAPPER_ENVELOPE, 'html');
+      $wrapper = $request->query->get(static::WRAPPER_ENVELOPE, $request->getRequestFormat());
       if (isset($this->mainContentRenderers[$wrapper])) {
         $renderer = $this->classResolver->getInstanceFromDefinition($this->mainContentRenderers[$wrapper]);
         $event->setResponse($renderer->renderResponse($result, $request, $this->routeMatch));
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 5b8b109..9c014e8 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -1786,11 +1786,6 @@ protected function drupalPostForm($path, $edit, $submit, array $options = array(
    * @see ajax.js
    */
   protected function drupalPostAjaxForm($path, $edit, $triggering_element, $ajax_path = NULL, array $options = array(), array $headers = array(), $form_html_id = NULL, $ajax_settings = NULL) {
-    if (isset($options['query'][MainContentViewSubscriber::WRAPPER_ENVELOPE])) {
-      $original_wrapper = $options['query'][MainContentViewSubscriber::WRAPPER_ENVELOPE];
-      unset($options['query'][MainContentViewSubscriber::WRAPPER_ENVELOPE]);
-    }
-
     // Get the content of the initial page prior to calling drupalPostForm(),
     // since drupalPostForm() replaces $this->content.
     if (isset($path)) {
@@ -1799,11 +1794,8 @@ protected function drupalPostAjaxForm($path, $edit, $triggering_element, $ajax_p
     $content = $this->content;
     $drupal_settings = $this->drupalSettings;
 
-    if (isset($original_wrapper)) {
-      $options['query'][MainContentViewSubscriber::WRAPPER_ENVELOPE] = $original_wrapper;
-    }
-    else {
-      $options['query'][MainContentViewSubscriber::WRAPPER_ENVELOPE] = 'drupal_ajax';
+    if (!preg_grep('/^Accept:/', $headers)) {
+      $headers[] = 'Accept: application/vnd.drupal-ajax';
     }
 
     // Get the Ajax settings bound to the triggering element.
@@ -1843,9 +1835,8 @@ protected function drupalPostAjaxForm($path, $edit, $triggering_element, $ajax_p
     // Unless a particular path is specified, use the one specified by the
     // Ajax settings, or else 'system/ajax'.
     if (!isset($ajax_path)) {
-      $ajax_path = isset($ajax_settings['url']) ? preg_replace('/^' . preg_quote($GLOBALS['base_path'], '/') . '/', '', $ajax_settings['url']) : 'system/ajax';
+      $ajax_path = isset($ajax_settings['url']) ? $ajax_settings['url'] : 'system/ajax';
     }
-    $ajax_path = $this->container->get('url_generator')->generateFromPath($ajax_path, $options);
 
     // Submit the POST request.
     $return = Json::decode($this->drupalPostForm(NULL, $edit, array('path' => $ajax_path, 'triggering_element' => $triggering_element), $options, $headers, $form_html_id, $extra_post));