gistfile1.txt

diff --git a/config/schema/embederator.schema.yml b/config/schema/embederator.schema.yml
index bd55b90..78a9222 100644
--- a/config/schema/embederator.schema.yml
+++ b/config/schema/embederator.schema.yml
@@ -30,3 +30,11 @@ embederator.embederator_type.*:
     wrapper_class:
       type: string
       label: 'Wrapper class for template'
+
+field.formatter.settings.embederator_default:
+  type: mapping
+  label: 'Embederator append unique DOM ID'
+  mapping:
+    append_unique_id:
+      type: boolean
+      label: 'Append unique DOM ID'
diff --git a/src/Plugin/Field/FieldFormatter/EmbederatorFormatter.php b/src/Plugin/Field/FieldFormatter/EmbederatorFormatter.php
index 305e71b..9872b63 100644
--- a/src/Plugin/Field/FieldFormatter/EmbederatorFormatter.php
+++ b/src/Plugin/Field/FieldFormatter/EmbederatorFormatter.php
@@ -5,6 +5,7 @@ namespace Drupal\embederator\Plugin\Field\FieldFormatter;
 use Drupal\Core\Field\FormatterBase;
 use Drupal\Core\Field\FieldItemListInterface;
 use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Component\Utility\Html;
 
 /**
  * Plugin implementation of the default embederator (token replacing) formatter.
@@ -41,6 +42,7 @@ class EmbederatorFormatter extends FormatterBase {
         try {
           $response = $client->request('GET', $url);
           $markup = (string) $response->getBody();
+          $markup = $this->uniquify($markup);
         }
         catch (Exception $e) {
           $markup = '<p>Unable to load ' . $url . '</p>';
@@ -58,6 +60,7 @@ class EmbederatorFormatter extends FormatterBase {
       $elements = [];
       foreach ($items as $delta => $item) {
         $markup = $token->replace($embed_pattern_field['value'], ['embederator' => $entity]);
+        $markup = $this->uniquify($markup);
         $elements[$delta] = [
           '#type' => 'processed_text',
           '#text' => $markup,
@@ -80,4 +83,66 @@ class EmbederatorFormatter extends FormatterBase {
             && ($field_definition->getName() == 'embed_id'));
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function settingsSummary() {
+    $summary = [];
+    if ($this->getSetting('append_unique_id')) {
+      $summary[] = $this->t('Append unique identifier to DOM IDs.');
+    }
+    return $summary;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function defaultSettings() {
+    return [
+      'append_unique_id' => FALSE,
+    ] + parent::defaultSettings();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function settingsForm(array $form, FormStateInterface $form_state) {
+    $element['append_unique_id'] = [
+      '#title' => $this->t('Append unique DOM ID'),
+      '#type' => 'checkbox',
+      '#default_value' => $this->getSetting('append_unique_id'),
+    ];
+
+    return $element;
+  }
+
+  /**
+   * Add a random suffix to ID attributes in DOM markup.
+   *
+   * See e.g., https://api.drupal.org/api/drupal/core%21modules%21filter%21src%21Plugin%21Filter%21FilterHtml.php/function/FilterHtml%3A%3AfilterAttributes/8.2.x.
+   */
+  protected function uniquify($html) {
+    if ($this->getSetting('append_unique_id')) {
+      $suffix = uniqid();
+
+      // Collect all the IDs and make their replacements.
+      $html_dom = Html::load($html);
+      $xpath = new \DOMXPath($html_dom);
+      foreach ($xpath->query('//*[@id]') as $element) {
+        $orig_id = $element->getAttribute('id');
+        $new_id = $orig_id . "-" . $suffix;
+        foreach ($xpath->query('//*[@for="' . $orig_id . '"]') as $for_element) {
+          $for_element->setAttribute('for', $new_id);
+        }
+        $element->setAttribute('id', $new_id);
+      }
+
+      $text = Html::serialize($html_dom);
+      $html = trim($text);
+    }
+
+    // Run the replacements on the markup.
+    return $html;
+  }
+
 }