core/modules/editor/src/Form/EditorImageDialog.php | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/core/modules/editor/src/Form/EditorImageDialog.php b/core/modules/editor/src/Form/EditorImageDialog.php
index 0d7e394..4d4d28d 100644
--- a/core/modules/editor/src/Form/EditorImageDialog.php
+++ b/core/modules/editor/src/Form/EditorImageDialog.php
@@ -95,11 +95,23 @@ public function buildForm(array $form, FormStateInterface $form_state, FilterFor
       $form['fid']['#required'] = FALSE;
     }
 
+    // The alt attribute is *required*, but we allow users to opt-in to empty
+    // alt attributes for the very rare edge cases where that is valid by
+    // specifying two double quotes as the alternative text in the dialog.
+    // However, that *is* stored as an empty alt attribute, so if we're editing
+    // an existing image (which means the src attribute is set) and its alt
+    // attribute is empty, then we show that as two double quotes in the dialog.
+    // @see https://www.drupal.org/node/2307647
+    $alt = isset($image_element['alt']) ? $image_element['alt'] : '';
+    if ($alt === '' && !empty($image_element['src'])) {
+      $alt = '""';
+    }
     $form['attributes']['alt'] = array(
       '#title' => $this->t('Alternative text'),
       '#type' => 'textfield',
       '#required' => TRUE,
-      '#default_value' => isset($image_element['alt']) ? $image_element['alt'] : '',
+      '#required_error' => $this->t('Alternative text is required.<br><em>(Only in rare cases should this be left empty. To create empty alternative text, please enter <tt>""</tt> — two double quotes without any content).'),
+      '#default_value' => $alt,
       '#maxlength' => 2048,
     );
     $form['dimensions'] = array(
@@ -201,6 +213,12 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
       $form_state->setValue(array('attributes', 'data-editor-file-uuid'), $file->uuid());
     }
 
+    // When the alt attribute is set to two double quotes, transform it to the
+    // empty string: two double quotes signify "empty alt attribute". See above.
+    if (trim($form_state->getValue(array('attributes', 'alt'))) === '""') {
+      $form_state->setValue(array('attributes', 'alt'), '');
+    }
+
     if (form_get_errors($form_state)) {
       unset($form['#prefix'], $form['#suffix']);
       $status_messages = array('#theme' => 'status_messages');