core/modules/ckeditor/js/plugins/drupalimage/plugin.js        | 3 +++
 core/modules/ckeditor/js/plugins/drupalimagecaption/plugin.js | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/core/modules/ckeditor/js/plugins/drupalimage/plugin.js b/core/modules/ckeditor/js/plugins/drupalimage/plugin.js
index 0ce9219..0112a4c 100644
--- a/core/modules/ckeditor/js/plugins/drupalimage/plugin.js
+++ b/core/modules/ckeditor/js/plugins/drupalimage/plugin.js
@@ -65,6 +65,9 @@
         });
 
         // Extend requiredContent & allowedContent.
+        // CKEDITOR.style is an immutable object: we cannot modify its
+        // definition to extend requiredContent. Hence we get the definition,
+        // modify it, and pass it to a new CKEDITOR.style instance.
         var requiredContent = widgetDefinition.requiredContent.getDefinition();
         requiredContent.attributes['data-entity-type'] = '';
         requiredContent.attributes['data-entity-uuid'] = '';
diff --git a/core/modules/ckeditor/js/plugins/drupalimagecaption/plugin.js b/core/modules/ckeditor/js/plugins/drupalimagecaption/plugin.js
index 731c982..22968b3 100644
--- a/core/modules/ckeditor/js/plugins/drupalimagecaption/plugin.js
+++ b/core/modules/ckeditor/js/plugins/drupalimagecaption/plugin.js
@@ -51,6 +51,9 @@
         }, true);
 
         // Extend requiredContent & allowedContent.
+        // CKEDITOR.style is an immutable object: we cannot modify its
+        // definition to extend requiredContent. Hence we get the definition,
+        // modify it, and pass it to a new CKEDITOR.style instance.
         var requiredContent = widgetDefinition.requiredContent.getDefinition();
         requiredContent.attributes['data-align'] = '';
         requiredContent.attributes['data-caption'] = '';