diff --git a/core/modules/aggregator/aggregator.module b/core/modules/aggregator/aggregator.module
index 1a5ed47..73e076c 100644
--- a/core/modules/aggregator/aggregator.module
+++ b/core/modules/aggregator/aggregator.module
@@ -544,7 +544,7 @@ function aggregator_filter_xss($value) {
 }
 
 /**
- * Implements hook_preprocess_HOOK() for block.tpl.php.
+ * Implements hook_preprocess_HOOK() for block.html.twig.
  */
 function aggregator_preprocess_block(&$variables) {
   if ($variables['block']->module == 'aggregator') {
diff --git a/core/modules/aggregator/aggregator.pages.inc b/core/modules/aggregator/aggregator.pages.inc
index bd8d793..d926179 100644
--- a/core/modules/aggregator/aggregator.pages.inc
+++ b/core/modules/aggregator/aggregator.pages.inc
@@ -330,6 +330,8 @@ function template_preprocess_aggregator_item(&$variables) {
   foreach ($item->categories as $category) {
     $variables['categories'][$category->cid] = l($category->title, 'aggregator/categories/' . $category->cid);
   }
+
+  $variables['attributes'] = new Attribute(array('class' => array('feed-item')));
 }
 
 /**
@@ -559,9 +561,14 @@ function template_preprocess_aggregator_summary_items(&$variables) {
 }
 
 /**
- * Processes variables for aggregator-summary-item.tpl.php.
+ * Processes variables for aggregator summary item templates.
+ *
+ * Default template: aggregator-summary-item.html.twig.
  *
- * @see aggregator-summary-item.tpl.php
+ * @param array $variables
+ *   An associative array containing:
+ *   - aggregator_item: The feed item.
+ *   - view_mode: How the item is being displayed.
  */
 function template_preprocess_aggregator_summary_item(&$variables) {
   $item = $variables['aggregator_item'];
@@ -614,4 +621,6 @@ function template_preprocess_aggregator_feed_source(&$variables) {
   if (user_access('administer news feeds')) {
     $variables['last_checked'] = l($variables['last_checked'], 'admin/config/services/aggregator');
   }
+
+  $variables['attributes'] = new Attribute(array('class' => array('feed-source')));
 }
diff --git a/core/modules/aggregator/templates/aggregator-feed-source.html.twig b/core/modules/aggregator/templates/aggregator-feed-source.html.twig
index 33fac7e..669eab6 100644
--- a/core/modules/aggregator/templates/aggregator-feed-source.html.twig
+++ b/core/modules/aggregator/templates/aggregator-feed-source.html.twig
@@ -20,7 +20,7 @@
  * @ingroup themeable
  */
 #}
-<div class="feed-source">
+<div{{ attributes}}>
   {{ source_icon }}
   {{ source_image }}
   <div class="feed-description">
diff --git a/core/modules/aggregator/templates/aggregator-item.html.twig b/core/modules/aggregator/templates/aggregator-item.html.twig
index 49ba80f..816ed66 100644
--- a/core/modules/aggregator/templates/aggregator-item.html.twig
+++ b/core/modules/aggregator/templates/aggregator-item.html.twig
@@ -18,7 +18,7 @@
  * @ingroup themeable
  */
 #}
-<div class="feed-item">
+<div{{ attributes }}>
   <h3 class="feed-item-title">
     <a href="{{ feed_url }}">{{ feed_title }}</a>
   </h3>