diff -u b/core/modules/system/tests/modules/ajax_test/src/Controller/AjaxTestController.php b/core/modules/system/tests/modules/ajax_test/src/Controller/AjaxTestController.php
--- b/core/modules/system/tests/modules/ajax_test/src/Controller/AjaxTestController.php
+++ b/core/modules/system/tests/modules/ajax_test/src/Controller/AjaxTestController.php
@@ -290,6 +290,7 @@
       'not-wrapped' => 'not-wrapped',
       'comment-not-wrapped' => '<!-- COMMENT --><div class="comment-not-wrapped">comment-not-wrapped</div>',
       'mixed' => ' foo <!-- COMMENT -->  foo bar<div class="a class"><p>some string</p></div> additional not wrapped strings, <!-- ANOTHER COMMENT --> <p>final string</p>',
+      'top-level-only' => '<div>element #1</div><div>element #2</div>',
       'inline' => 'inline <div>BLOCK LEVEL<script> var test;</script></div>',
       'empty' => '',
     ];
diff -u b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxTest.php b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxTest.php
--- b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxTest.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxTest.php
@@ -138,6 +138,13 @@
         'render_type' => 'mixed',
         'expected' => '<div class="processed">foo <!-- COMMENT -->  foo bar<div class="a class"><p>some string</p></div> additional not wrapped strings, <!-- ANOTHER COMMENT --> <p>final string</p></div>',
       ],
+      // Test that when the response has only top-level node elements, they
+      // are processed properly without extra wrapping.
+      [
+        'method' => 'html',
+        'render_type' => 'top-level-only',
+        'expected' => '<div class="processed">element #1</div><div class="processed">element #2</div>',
+      ],
       // Test that inline response data.
       [
         'render_type' => 'inline',