diff -u b/core/modules/aggregator/aggregator.pages.inc b/core/modules/aggregator/aggregator.pages.inc
--- b/core/modules/aggregator/aggregator.pages.inc
+++ b/core/modules/aggregator/aggregator.pages.inc
@@ -53,7 +53,7 @@
  *   The category for which to list all of the aggregated items.
  *
  * @return string
- *   The rendered list of items for the feed.
+ *   The render array with list of items for the feed.
  *
  * @see aggregator_menu()
  * @ingroup forms
diff -u b/core/modules/aggregator/aggregator.routing.yml b/core/modules/aggregator/aggregator.routing.yml
--- b/core/modules/aggregator/aggregator.routing.yml
+++ b/core/modules/aggregator/aggregator.routing.yml
@@ -72,5 +72,5 @@
   pattern: '/aggregator/categories/{cid}'
   defaults:
-    _controller: '\Drupal\aggregator\Controller\AggregatorController::category'
+    _content: '\Drupal\aggregator\Controller\AggregatorController::category'
   requirements:
     _permission: 'access news feeds'
diff -u b/core/modules/aggregator/lib/Drupal/aggregator/Controller/AggregatorController.php b/core/modules/aggregator/lib/Drupal/aggregator/Controller/AggregatorController.php
--- b/core/modules/aggregator/lib/Drupal/aggregator/Controller/AggregatorController.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Controller/AggregatorController.php
@@ -329,22 +329,17 @@
    * @param int $cid
    *   The category id for which to list all of the aggregated items.
    *
-   * @return string
-   *   The rendered list of items for the feed.
+   * @return array
+   *   The render array with list of items for the feed.
    */
   public function category($cid) {
     $category = $this->database->query('SELECT cid, title FROM {aggregator_category} WHERE cid = :cid', array(':cid' => $cid))->fetchObject();
     drupal_set_title($category->title);
-    drupal_add_feed('aggregator/rss/' . $category->cid, $this->configFactory->get('system.site')->get('name') . ' ' . t('aggregator - @title', array('@title' => $category->title)));
 
     // @todo move included functions to controller.
     module_load_include('inc', 'aggregator', 'aggregator.pages');
 
-    // It is safe to include the cid in the query because it's loaded from the
-    // database by aggregator_category_load().
-    $items = aggregator_load_feed_items('category', $category);
-
-    return _aggregator_page_list($items, $cid);
+    return aggregator_page_category($category);
   }
 
 }