diff --git a/core/lib/Drupal/Core/ContentNegotiation.php b/core/lib/Drupal/Core/ContentNegotiation.php
index 8aa7e48..10237ab 100644
--- a/core/lib/Drupal/Core/ContentNegotiation.php
+++ b/core/lib/Drupal/Core/ContentNegotiation.php
@@ -37,10 +37,6 @@ public function getContentType(Request $request) {
       return $request->query->get('_format');
     }
 
-    if ($request->isXmlHttpRequest()) {
-      return 'ajax';
-    }
-
     // Do HTML last so that it always wins.
     return 'html';
   }
diff --git a/core/tests/Drupal/Tests/Core/ContentNegotiationTest.php b/core/tests/Drupal/Tests/Core/ContentNegotiationTest.php
index 9c54eae..b5f9d3f 100644
--- a/core/tests/Drupal/Tests/Core/ContentNegotiationTest.php
+++ b/core/tests/Drupal/Tests/Core/ContentNegotiationTest.php
@@ -73,7 +73,7 @@ public function testUnknowContentTypeButAjaxRequest() {
     $request = new Request();
     $request->headers->set('X-Requested-With', 'XMLHttpRequest');
 
-    $this->assertSame('ajax', $this->contentNegotiation->getContentType($request));
+    $this->assertSame('html', $this->contentNegotiation->getContentType($request));
   }
 
 }