core/modules/ckeditor5/src/HTMLRestrictions.php                     | 6 ++++++
 .../ckeditor5/tests/src/FunctionalJavascript/SourceEditingTest.php  | 6 +++---
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/core/modules/ckeditor5/src/HTMLRestrictions.php b/core/modules/ckeditor5/src/HTMLRestrictions.php
index d6987cbd91..daeb1a4812 100644
--- a/core/modules/ckeditor5/src/HTMLRestrictions.php
+++ b/core/modules/ckeditor5/src/HTMLRestrictions.php
@@ -817,6 +817,12 @@ public function toGeneralHtmlSupportConfig(): array {
             $to_allow['classes'] = $value;
             continue;
           }
+          // If a single attribute value is allowed, it must be TRUE (see the
+          // assertion above). Otherwise, it must be an array of strings (see
+          // the assertion above), which lists all allowed attribute values. To
+          // be able to configure GHS to a range of values, we need to use a
+          // regular expression.
+          // @todo Expand to support partial wildcards in https://www.drupal.org/project/drupal/issues/3260853.
           $to_allow['attributes'][$name] = is_array($value)
             ? ['regexp' => ['pattern' => '/^(' . implode('|', $value) . ')$/']]
             : $value;
diff --git a/core/modules/ckeditor5/tests/src/FunctionalJavascript/SourceEditingTest.php b/core/modules/ckeditor5/tests/src/FunctionalJavascript/SourceEditingTest.php
index 17435b2292..d9a057fcb6 100644
--- a/core/modules/ckeditor5/tests/src/FunctionalJavascript/SourceEditingTest.php
+++ b/core/modules/ckeditor5/tests/src/FunctionalJavascript/SourceEditingTest.php
@@ -26,7 +26,7 @@ class SourceEditingTest extends CKEditor5TestBase {
   protected $adminUser;
 
   /**
-   * A host entity with a body field to embed images in.
+   * A host entity with a body field whose text to edit with CKEditor 5.
    *
    * @var \Drupal\node\NodeInterface
    */
@@ -44,7 +44,7 @@ class SourceEditingTest extends CKEditor5TestBase {
   /**
    * {@inheritdoc}
    */
-  protected $defaultTheme = 'classy';
+  protected $defaultTheme = 'stark';
 
   /**
    * {@inheritdoc}
@@ -100,7 +100,7 @@ function (ConstraintViolation $v) {
       'bypass node access',
     ]);
 
-    // Create a sample host entity to embed images in.
+    // Create a sample host entity to test CKEditor 5.
     $this->host = $this->createNode([
       'type' => 'page',
       'title' => 'Animals with strange names',