diff --cc core/modules/simpletest/src/WebTestBase.php
index 50f9533,32e6e5e..0000000
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@@ -11,13 -10,19 +11,15 @@@ use Drupal\block\Entity\Block
  use Drupal\Component\FileCache\FileCacheFactory;
  use Drupal\Component\Serialization\Json;
  use Drupal\Component\Serialization\Yaml;
 -use Drupal\Component\Utility\Crypt;
  use Drupal\Component\Utility\Html;
  use Drupal\Component\Utility\NestedArray;
- use Drupal\Component\Utility\SafeMarkup;
+ use Drupal\Component\Utility\UrlHelper;
  use Drupal\Core\Cache\Cache;
+ use Drupal\Component\Utility\SafeMarkup;
 -use Drupal\Core\DependencyInjection\YamlFileLoader;
 -use Drupal\Core\DrupalKernel;
  use Drupal\Core\Database\Database;
 -use Drupal\Core\Database\ConnectionNotDefinedException;
 +use Drupal\Core\DrupalKernel;
  use Drupal\Core\Entity\EntityInterface;
+ use Drupal\Core\EventSubscriber\MainContentViewSubscriber;
 -use Drupal\Core\Language\LanguageInterface;
  use Drupal\Core\Render\Element;
  use Drupal\Core\Session\AccountInterface;
  use Drupal\Core\Session\AnonymousUserSession;
@@@ -1699,8 -1843,21 +1708,26 @@@ abstract class WebTestBase extends Test
      // 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']) ? $ajax_settings['url'] : 'system/ajax';
+       if (isset($ajax_settings['url'])) {
++        // In order to allow to set for example the wrapper envelope query
++        // parameter we need to get the system path again.
+         $parsed_url = UrlHelper::parse($ajax_settings['url']);
+         $options['query'] = $parsed_url['query'] + $options['query'];
+         $options += ['fragment' => $parsed_url['fragment']];
++
++        // We know that $parsed_url['path'] is already with the base path
++        // attached.
+         $ajax_path = preg_replace(
 -          '/^' . preg_quote($GLOBALS['base_path'], '/') . '/',
++          '/^' . preg_quote(base_path(), '/') . '/',
+           '',
+           $parsed_url['path']
+         );
+       }
+       else {
+         $ajax_path = 'system/ajax';
+       }
      }
+     $ajax_path = $this->container->get('unrouted_url_assembler')->assemble('base://' . $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));
diff --git a/core/lib/Drupal/Core/ContentNegotiation.php b/core/lib/Drupal/Core/ContentNegotiation.php
index 3322af8..e45c92d 100644
--- a/core/lib/Drupal/Core/ContentNegotiation.php
+++ b/core/lib/Drupal/Core/ContentNegotiation.php
@@ -38,10 +38,9 @@ public function getContentType(Request $request) {
 
     // Check all formats, if priority format is found return it.
     $first_found_format = FALSE;
-    $priority = array('html');
     foreach ($request->getAcceptableContentTypes() as $mime_type) {
       $format = $request->getFormat($mime_type);
-      if (in_array($format, $priority, TRUE)) {
+      if ($format === 'html') {
         return $format;
       }
       if (!is_null($format) && !$first_found_format) {