diff --git a/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php b/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
index 2701dcb..d9df046 100644
--- a/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
+++ b/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
@@ -98,7 +98,8 @@ public function testForm() {
 
     // Test drupalPostForm().
     $edit = ['bananas' => 'red'];
-    $this->drupalPostForm('form-test/object-builder', $edit, 'Save');
+    $result = $this->drupalPostForm('form-test/object-builder', $edit, 'Save');
+    $this->assertSame($this->getSession()->getPage()->getContent(), $result);
     $value = $config_factory->get('form_test.object')->get('bananas');
     $this->assertSame('red', $value);
 
diff --git a/core/tests/Drupal/Tests/BrowserTestBase.php b/core/tests/Drupal/Tests/BrowserTestBase.php
index bd50c23..91a10cd 100644
--- a/core/tests/Drupal/Tests/BrowserTestBase.php
+++ b/core/tests/Drupal/Tests/BrowserTestBase.php
@@ -920,6 +920,9 @@ protected function submitForm(array $edit, $submit, $form_html_id = NULL) {
    *   POST data.
    * @param array $options
    *   Options to be forwarded to the url generator.
+   *
+   * @return string
+   *   The HTML of the rendered page.
    */
   protected function drupalPostForm($path, $edit, $submit, array $options = []) {
     if (is_object($submit)) {
@@ -938,6 +941,8 @@ protected function drupalPostForm($path, $edit, $submit, array $options = []) {
     }
 
     $this->submitForm($edit, $submit);
+
+    return $this->getSession()->getPage()->getContent();
   }
 
   /**