diff --git a/core/modules/filter/src/Tests/FilterFormTest.php b/core/modules/filter/src/Tests/FilterFormTest.php
index 460b8a9..dde1698 100644
--- a/core/modules/filter/src/Tests/FilterFormTest.php
+++ b/core/modules/filter/src/Tests/FilterFormTest.php
@@ -93,6 +93,9 @@ protected function doFilterFormTestAsAdmin() {
     // 'filter_test' as the default value in this case.
     $this->assertOptions('edit-all-formats-default-format--2', $formats, 'filter_test');
     $this->assertEnabledTextarea('edit-all-formats-default-missing-value');
+    // If a missing format is set as the default, administrators must select a
+    // valid replacement format.
+    $this->assertRequiredSelectAndOptions('edit-all-formats-default-missing-format--2', $formats);
 
     // Test a text format element with a predefined list of formats.
     $formats = ['full_html', 'filter_test'];
@@ -101,7 +104,9 @@ protected function doFilterFormTestAsAdmin() {
     $this->assertEnabledTextarea('edit-restricted-formats-default-value');
     $this->assertOptions('edit-restricted-formats-default-format--2', $formats, 'full_html');
     $this->assertEnabledTextarea('edit-restricted-formats-default-missing-value');
+    $this->assertRequiredSelectAndOptions('edit-restricted-formats-default-missing-format--2', $formats);
     $this->assertEnabledTextarea('edit-restricted-formats-default-disallowed-value');
+    $this->assertRequiredSelectAndOptions('edit-restricted-formats-default-disallowed-format--2', $formats);
 
     // Test a text format element with a fixed format.
     $formats = ['filter_test'];
@@ -113,7 +118,9 @@ protected function doFilterFormTestAsAdmin() {
     // If the select has a missing or disallowed format, administrators must
     // explicitly choose the format.
     $this->assertEnabledTextarea('edit-single-format-default-missing-value');
+    $this->assertRequiredSelectAndOptions('edit-single-format-default-missing-format--2', $formats);
     $this->assertEnabledTextarea('edit-single-format-default-disallowed-value');
+    $this->assertRequiredSelectAndOptions('edit-single-format-default-disallowed-format--2', $formats);
   }
 
   /**
@@ -243,7 +250,7 @@ protected function assertOptions($id, array $expected_options, $selected) {
    *   TRUE if the assertion passed; FALSE otherwise.
    */
   protected function assertRequiredSelectAndOptions($id, array $options) {
-    $select = $this->xpath('//select[@id=:id and contains(@required, "required")]', [
+    $select = $this->xpath('//select[@id=:id and contains(@class, "required")]', [
       ':id' => $id,
     ]);
     $select = reset($select);
diff --git a/core/modules/system/src/Tests/Form/FormTest.php b/core/modules/system/src/Tests/Form/FormTest.php
index b0e6a9d..a61c818 100644
--- a/core/modules/system/src/Tests/Form/FormTest.php
+++ b/core/modules/system/src/Tests/Form/FormTest.php
@@ -263,6 +263,7 @@ public function testInputWithInvalidToken() {
       'textarea' => $this->randomString() . "\n",
       'form_token' => 'invalid token',
     ];
+    $this->drupalPostForm(Url::fromRoute('form_test.required'), $edit, 'Submit');
     $this->assertFieldByXpath('//div[contains(@class, "error")]', NULL, 'Error message is displayed with invalid token even when required fields are filled.');
     $this->assertText('The form has become outdated. Copy any unsaved work in the form below');
     $this->assertFieldByName('textfield', $edit['textfield']);
diff --git a/core/modules/system/tests/modules/form_test/form_test.routing.yml b/core/modules/system/tests/modules/form_test/form_test.routing.yml
index 099d093..4233b1b 100644
--- a/core/modules/system/tests/modules/form_test/form_test.routing.yml
+++ b/core/modules/system/tests/modules/form_test/form_test.routing.yml
@@ -393,6 +393,14 @@ form_test.checkboxes_zero:
   requirements:
     _access: 'TRUE'
 
+form_test.required:
+  path: '/form-test/required-fields'
+  defaults:
+    _form: '\Drupal\form_test\Form\FormTestRequiredFieldsForm'
+    _title: 'Required'
+  requirements:
+    _access: 'TRUE'
+
 form_test.button_class:
   path: '/form-test/button-class'
   defaults:
diff --git a/core/modules/system/tests/modules/form_test/src/Form/FormTestRequiredFieldsForm.php b/core/modules/system/tests/modules/form_test/src/Form/FormTestRequiredFieldsForm.php
new file mode 100644
index 0000000..bdbc343
--- /dev/null
+++ b/core/modules/system/tests/modules/form_test/src/Form/FormTestRequiredFieldsForm.php
@@ -0,0 +1,44 @@
+<?php
+
+namespace Drupal\form_test\Form;
+
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * Builds a form to test required fields.
+ */
+class FormTestRequiredFieldsForm extends FormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'form_test_required_fields';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    foreach (['textfield', 'textarea', 'password'] as $type) {
+      $form[$type] = [
+        '#type' => $type,
+        '#required' => TRUE,
+        '#title' => $type,
+      ];
+    }
+    $form['submit'] = [
+      '#type' => 'submit',
+      '#value' => 'Submit',
+    ];
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+  }
+
+}