diff --git a/core/modules/ckeditor5/tests/src/Kernel/SmartDefaultSettingsTest.php b/core/modules/ckeditor5/tests/src/Kernel/SmartDefaultSettingsTest.php
index 06243ff6d44e97060903c36ce1dd4541c0d014f1..e79413205d6d158753fc05841d2b0119290d07ef 100644
--- a/core/modules/ckeditor5/tests/src/Kernel/SmartDefaultSettingsTest.php
+++ b/core/modules/ckeditor5/tests/src/Kernel/SmartDefaultSettingsTest.php
@@ -53,6 +53,10 @@ class SmartDefaultSettingsTest extends KernelTestBase {
     'editor',
     'filter',
     'user',
+    // For being able to test media_embed + Media button in CKE4/CKE5.
+    'media',
+    'media_library',
+    'views',
   ];
 
   /**
@@ -109,6 +113,26 @@ protected function setUp(): void {
       Yaml::parseFile('core/profiles/standard/config/install/editor.editor.basic_html.yml')
     )->save();
 
+    $basic_html_format_with_media_embed = $basic_html_format;
+    $basic_html_format_with_media_embed['name'] .= ' (with Media Embed support)';
+    $basic_html_format_with_media_embed['format'] = 'basic_html_with_media_embed';
+    // Add media_embed filter, update filter_html filter settings.
+    $basic_html_format_with_media_embed['filters']['media_embed'] = ['status' => TRUE];
+    $new_value = $current_value . ' <drupal-media data-entity-type data-entity-uuid data-align data-caption alt>';
+    NestedArray::setValue($basic_html_format_with_media_embed, $allowed_html_parents, $new_value);
+    FilterFormat::create($basic_html_format_with_media_embed)->save();
+    $basic_html_editor_with_media_embed = Editor::create(
+      ['format' => 'basic_html_with_media_embed']
+      +
+      Yaml::parseFile('core/profiles/standard/config/install/editor.editor.basic_html.yml')
+    );
+    $settings = $basic_html_editor_with_media_embed->getSettings();
+    // Add "insert media from library" button to CKEditor 4 configuration, the
+    // pre-existing toolbar item group labeled "Media".
+    $settings['toolbar']['rows'][0][3]['items'][] = 'DrupalMediaLibrary';
+    $basic_html_editor_with_media_embed->setSettings($settings);
+    $basic_html_editor_with_media_embed->save();
+
     $filter_plugin_manager = $this->container->get('plugin.manager.filter');
     FilterFormat::create([
       'format' => 'filter_only__filter_html',
@@ -517,6 +541,33 @@ public function provider() {
         ]),
     ];
 
+    yield "basic_html with media_embed added => <drupal-media> needed => supported through sourceEditing (3 upgrade messages)" => [
+      'format_id' => 'basic_html_with_media_embed',
+      'filters_to_drop' => $basic_html_test_case['filters_to_drop'],
+      'expected_ckeditor5_settings' => [
+        'toolbar' => [
+          'items' => array_merge(
+            array_slice($basic_html_test_case['expected_ckeditor5_settings']['toolbar']['items'], 0, 10),
+            ['drupalMedia'],
+            array_slice($basic_html_test_case['expected_ckeditor5_settings']['toolbar']['items'], 10),
+          ),
+        ],
+        'plugins' => [
+          'ckeditor5_sourceEditing' => [
+            'allowed_tags' => array_merge(
+              $basic_html_test_case['expected_ckeditor5_settings']['plugins']['ckeditor5_sourceEditing']['allowed_tags'],
+              ['<drupal-media data-align data-caption>'],
+            ),
+          ],
+        ] + $basic_html_test_case['expected_ckeditor5_settings']['plugins'],
+      ],
+      'expected_superset' => $basic_html_test_case['expected_superset'],
+      'expected_fundamental_compatibility_violations' => $basic_html_test_case['expected_fundamental_compatibility_violations'],
+      'expected_messages' => array_merge($basic_html_test_case['expected_messages'], [
+        "This format's HTML filters includes plugins that support the following tags, but not some of their attributes. To ensure these attributes remain supported by this text format, the following were added to the Source Editing plugin's <em>Manually editable HTML tags</em>: &lt;a hreflang&gt; &lt;blockquote cite&gt; &lt;ul type&gt; &lt;ol start type&gt; &lt;h2 id&gt; &lt;h3 id&gt; &lt;h4 id&gt; &lt;h5 id&gt; &lt;h6 id&gt; &lt;drupal-media data-align data-caption&gt;.",
+      ]),
+    ];
+
     yield "restricted_html can be switched to CKEditor 5 after dropping the two markup-creating filters (3 upgrade messages)" => [
       'format_id' => 'restricted_html',
       'filters_to_drop' => [