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 2250a0b..b688779 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
@@ -489,3 +489,11 @@ form_test.get_form:
     _form: '\Drupal\form_test\Form\FormTestGetForm'
   requirements:
     _access: 'TRUE'
+
+form_test.optional_container:
+  path: '/form-test/optional-container'
+  defaults:
+    _form: '\Drupal\form_test\Form\FormTestOptionalContainerForm'
+    _title: 'Optional container testing'
+  requirements:
+    _access: 'TRUE'
diff --git a/core/modules/system/tests/modules/form_test/src/Form/FormTestOptionalContainerForm.php b/core/modules/system/tests/modules/form_test/src/Form/FormTestOptionalContainerForm.php
new file mode 100644
index 0000000..74cb47c
--- /dev/null
+++ b/core/modules/system/tests/modules/form_test/src/Form/FormTestOptionalContainerForm.php
@@ -0,0 +1,72 @@
+<?php
+
+namespace Drupal\form_test\Form;
+
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * Builds a simple form to test the #optional property on #type 'container'.
+ */
+class FormTestOptionalContainerForm extends FormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'form_test_optional_container';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    // Empty containers.
+    $form['empty_optional'] = [
+      '#type' => 'container',
+      '#attributes' => ['class' => ['empty_optional']],
+      '#optional' => TRUE,
+    ];
+    $form['empty_optional']['child'] = [
+      '#type' => 'markup',
+      '#vaule' => ''
+    ];
+    $form['empty_nonoptional'] = [
+      '#type' => 'container',
+      '#attributes' => ['class' => ['empty_nonoptional']],
+      '#optional' => FALSE,
+    ];
+    $form['empty_nonoptional']['child'] = [
+      '#type' => 'markup',
+      '#vaule' => ''
+    ];
+    // Non-empty containers
+    $form['nonempty_optional'] = [
+      '#type' => 'container',
+      '#attributes' => ['class' => ['nonempty_optional']],
+      '#optional' => TRUE,
+    ];
+    $form['nonempty_optional']['child'] = [
+      '#type' => 'markup',
+      '#vaule' => '<b>nonempty_optional value</b>'
+    ];
+    $form['nonempty_nonoptional'] = [
+      '#type' => 'container',
+      '#attributes' => ['class' => ['nonempty_nonoptional']],
+      '#optional' => FALSE,
+    ];
+    $form['nonempty_nonoptional']['child'] = [
+      '#type' => 'markup',
+      '#vaule' => '<b>nonempty_nonoptional value</b>'
+    ];
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+  }
+
+}
diff --git a/core/modules/system/tests/src/Functional/Form/ElementsContainerTest.php b/core/modules/system/tests/src/Functional/Form/ElementsContainerTest.php
new file mode 100644
index 0000000..64deb40
--- /dev/null
+++ b/core/modules/system/tests/src/Functional/Form/ElementsContainerTest.php
@@ -0,0 +1,45 @@
+<?php
+
+namespace Drupal\Tests\system\Functional\Form;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests the container form element for expected behavior.
+ *
+ * @group Form
+ */
+class ElementsContainerTest extends BrowserTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = ['form_test'];
+
+  /**
+   * Tests the #optional container property.
+   */
+  public function testOptionalContainerElements() {
+    $this->drupalGet('form-test/optional-container');
+
+    // Assert that an empty optional container is not shown.
+    $elements =$this->assertSession()->elementExists('css', 'div.empty_optional');
+    $this->assertTrue(count($elements) == 0);
+
+    // empty_nonoptional
+    $elements = $this->assertSession()->elementExists('css', 'div.empty_nonoptional');
+    $this->assertTrue(count($elements) == 1);
+
+    // nonempty_optional
+    $elements = $this->assertSession()->elementExists('css', 'div.nonempty_optional');
+    $this->assertTrue(count($elements) == 1);
+
+    // nonempty_nonoptional
+    $elements = $this->assertSession()->elementExists('css', 'div.nonempty_nonoptional');
+    $this->assertTrue(count($elements) == 1);
+
+  }
+
+}
diff --git a/core/tests/Drupal/Tests/Core/Render/Element/ContainerTest.php b/core/tests/Drupal/Tests/Core/Render/Element/ContainerTest.php
deleted file mode 100644
index 3f71cdc..0000000
--- a/core/tests/Drupal/Tests/Core/Render/Element/ContainerTest.php
+++ /dev/null
@@ -1,63 +0,0 @@
-<?php
-
-namespace Drupal\Tests\Core\Render\Element;
-
-use Drupal\Tests\Core\Render\RendererTestBase;
-use Drupal\Core\Render\Element\Container;
-
-/**
- * @coversDefaultClass \Drupal\Core\Render\Element\Container
- * @group Render
- */
-class ContainerTest extends RendererTestBase {
-
-  /**
-   * @covers ::preRenderContainer
-   * @dataProvider providerPreRenderContainer
-   */
-  public function testPreRenderContainer($element, $expected) {
-    $result = Container::preRenderContainer($element);
-    $this->assertEquals($expected, (string) $this->renderer->renderRoot($result));
-  }
-
-  /**
-   * Data provider for preRenderContainer test.
-   */
-  public function providerPreRenderContainer() {
-    $containers = [];
-
-    // Empty optional container.
-    $element = [
-      '#type' => 'container',
-      '#optional' => TRUE,
-    ];
-    $element['child1'] = ['#markup' => ''];
-    $containers['empty-optional'] = [$element, ''];
-
-    // Non-empty optional container.
-    $element = [
-      '#optional' => TRUE,
-    ];
-    $element['child1'] = ['#markup' => '<b>nested value</b>'];
-    $containers['non-empty-optional'] = [$element, '<div><b>nested value</b></div>'];
-
-    // Empty non-optional container.
-    $element = [
-      '#type' => 'container',
-      '#optional' => FALSE,
-    ];
-    $element['child1'] = ['#markup' => ''];
-    $containers['empty-non-optional'] = [$element, '<div></div>'];
-
-    // Non-empty non-optional container.
-    $element = [
-      '#type' => 'container',
-      '#optional' => FALSE,
-    ];
-    $element['child1'] = ['#markup' => '<b>nested value</b>'];
-    $containers['non-empty-non-optional'] = [$element, '<div><b>nested value</b></div>'];
-
-    return $containers;
-  }
-
-}