core/modules/editor/src/Form/EditorImageDialog.php | 1 +
 1 file changed, 1 insertion(+)

diff --git a/core/modules/editor/src/Form/EditorImageDialog.php b/core/modules/editor/src/Form/EditorImageDialog.php
index 0d7e394..9286b61 100644
--- a/core/modules/editor/src/Form/EditorImageDialog.php
+++ b/core/modules/editor/src/Form/EditorImageDialog.php
@@ -97,6 +97,7 @@ public function buildForm(array $form, FormStateInterface $form_state, FilterFor
 
     $form['attributes']['alt'] = array(
       '#title' => $this->t('Alternative text'),
+      '#placeholder' => $this->t('Short description of the image, for visually impaired users.'),
       '#type' => 'textfield',
       '#required' => TRUE,
       '#default_value' => isset($image_element['alt']) ? $image_element['alt'] : '',