diff --git a/core/modules/system/tests/modules/form_test/src/Form/FormTestSelectForm.php b/core/modules/system/tests/modules/form_test/src/Form/FormTestSelectForm.php
index 20242ec2eb..a922d2a86f 100644
--- a/core/modules/system/tests/modules/form_test/src/Form/FormTestSelectForm.php
+++ b/core/modules/system/tests/modules/form_test/src/Form/FormTestSelectForm.php
@@ -115,6 +115,14 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       '#multiple' => TRUE,
     ];
 
+    $form['opt_groups'] = [
+      '#type' => 'select',
+      '#options' => [
+        'optgroup_one' => ['one' => 'one', 'two' => 'two', 'three' => 'three', 'four' => '<strong>four</strong>'],
+        'optgroup_two' => ['five' => 'five', 'six' => 'six'],
+      ],
+    ];
+
     $form['submit'] = ['#type' => 'submit', '#value' => 'Submit'];
     return $form;
   }
diff --git a/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php b/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
index d11ef7686f..be1538785f 100644
--- a/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
+++ b/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\FunctionalTests;
 
+use Behat\Mink\Element\NodeElement;
 use Behat\Mink\Exception\ExpectationException;
 use Behat\Mink\Selector\Xpath\Escaper;
 use Drupal\Component\Render\FormattableMarkup;
@@ -812,4 +813,21 @@ protected function getRawContent() {
     return $this->getSession()->getPage()->getContent();
   }
 
+  /**
+   * Get all option elements, including nested options, in a select.
+   *
+   * @param \Behat\Mink\Element\NodeElement $element
+   *   The element for which to get the options.
+   *
+   * @return \Behat\Mink\Element\NodeElement[]
+   *   Option elements in select.
+   *
+   * @deprecated Scheduled for removal in Drupal 9.0.0.
+   *   Use $element->findAll('xpath', 'option') instead.
+   */
+  protected function getAllOptions(NodeElement $element) {
+    @trigger_error('AssertLegacyTrait::getAllOptions() is scheduled for removal in Drupal 9.0.0. Use $element->findAll(\'xpath\', \'option\') instead.', E_USER_DEPRECATED);
+    return $element->findAll('xpath', '//option');
+  }
+
 }
diff --git a/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php b/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
index c9f621a796..0a8eb29d84 100644
--- a/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
+++ b/core/tests/Drupal/FunctionalTests/BrowserTestBaseTest.php
@@ -428,6 +428,10 @@ public function testLegacyFieldAssertsForOptions() {
     catch (\PHPUnit_Framework_ExpectationFailedException $e) {
       $this->pass($e->getMessage());
     }
+
+    // Test \Drupal\FunctionalTests\AssertLegacyTrait::getAllOptions.
+    $this->drupalGet('/form-test/select');
+    $this->assertCount(6, $this->getAllOptions($this->cssSelect('select[name="opt_groups"]')[0]));
   }
 
   /**