diff --git a/core/modules/ckeditor5/src/HTMLRestrictions.php b/core/modules/ckeditor5/src/HTMLRestrictions.php
index 3a2d7e886c5..25503760afc 100644
--- a/core/modules/ckeditor5/src/HTMLRestrictions.php
+++ b/core/modules/ckeditor5/src/HTMLRestrictions.php
@@ -768,8 +768,11 @@ final class HTMLRestrictions {
     }
     // Make sure the order of the union array matches the order of the keys in
     // the arrays provided.
-    $keys_order = array_merge($array1_keys, $array2_keys);
-    return array_merge(array_flip($keys_order), $union);
+    $ordered = [];
+    foreach (array_merge($array1_keys, $array2_keys) as $key) {
+      $ordered[$key] = $union[$key];
+    }
+    return $ordered;
   }
 
   /**
diff --git a/core/modules/ckeditor5/tests/src/Unit/HTMLRestrictionsTest.php b/core/modules/ckeditor5/tests/src/Unit/HTMLRestrictionsTest.php
index dec20207f26..41179261df8 100644
--- a/core/modules/ckeditor5/tests/src/Unit/HTMLRestrictionsTest.php
+++ b/core/modules/ckeditor5/tests/src/Unit/HTMLRestrictionsTest.php
@@ -997,6 +997,13 @@ class HTMLRestrictionsTest extends UnitTestCase {
       'intersection' => 'a',
       'union' => 'b',
     ];
+    yield 'attribute restrictions are the same: <ol type="1"> vs <ol type="1">' => [
+      'a' => new HTMLRestrictions(['ol' => ['type' => ['1' => TRUE]]]),
+      'b' => new HTMLRestrictions(['ol' => ['type' => ['1' => TRUE]]]),
+      'diff' => HTMLRestrictions::emptySet(),
+      'intersection' => 'a',
+      'union' => 'a',
+    ];
 
     // Complex cases.
     yield 'attribute restrictions are different: <a hreflang="en"> vs <strong>' => [