Wim Leers
--- a/core/modules/ckeditor5/tests/src/Functional/Update/CKEditor5UpdateIOlStartReversed.php 0 → 100644
+++ b/core/modules/ckeditor5/tests/src/Functional/Update/CKEditor5UpdateIOlStartReversed.php 0 → 100644
+    $settings = $before['test_format_list_ol_start']->getSettings();
+    $this->assertArrayHasKey('ckeditor5_list', $settings['plugins']);
+    $this->assertSame(['<ol start foo>'], $settings['plugins']['ckeditor5_sourceEditing']['allowed_tags']);
+
+    // test_text_format before: not using the List plugin.
+    $settings = $before['test_text_format']->getSettings();
+    $this->assertArrayNotHasKey('ckeditor5_list', $settings['plugins']);
+
+    $this->runUpdates();
+
+    $after = Editor::loadMultiple();
+
+    // Basic HTML after: reversed=FALSE, startIndex=FALSE, Source Editing
+    // configuration unchanged.
+    $settings = $after['basic_html']->getSettings();
+    $this->assertSame(['reversed' => FALSE, 'startIndex' => FALSE], $settings['plugins']['ckeditor5_list']);
--- a/core/modules/ckeditor5/tests/src/Functional/Update/CKEditor5UpdateIOlStartReversed.php 0 → 100644
+++ b/core/modules/ckeditor5/tests/src/Functional/Update/CKEditor5UpdateIOlStartReversed.php 0 → 100644
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Tests\ckeditor5\Functional\Update;
+
+use Drupal\ckeditor5\HTMLRestrictions;
+use Drupal\editor\Entity\Editor;
+use Drupal\FunctionalTests\Update\UpdatePathTestBase;
+
+/**
+ * @covers ckeditor5_post_update_list_start_reversed
+ * @group Update
+ * @group ckeditor5
+ */
+class CKEditor5UpdateIOlStartReversed extends UpdatePathTestBase {
--- a/core/profiles/standard/config/install/editor.editor.basic_html.yml
+++ b/core/profiles/standard/config/install/editor.editor.basic_html.yml
        - '<a hreflang>'
        - '<blockquote cite>'
        - '<ul type>'
-        - '<ol start type>'
+        - '<ol type>'
--- a/core/modules/ckeditor5/tests/src/Kernel/ValidatorsTest.php
+++ b/core/modules/ckeditor5/tests/src/Kernel/ValidatorsTest.php
      ],
      'violations' => [],
    ];
+    $data['INVALID: SourceEditing plugin configuration: <ol start type> must not be allowed because List can generate <ol start>'] = [
+      'settings' => [
+        'toolbar' => [
+          'items' => [
+            'numberedList',
+            'sourceEditing',
+          ],
+        ],
+        'plugins' => [
+          'ckeditor5_list' => [
+            'properties' => [
+              'reversed' => TRUE,
+              'startIndex' => TRUE,