diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 34dbdc2..b454e71 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1298,8 +1298,6 @@ function _template_preprocess_default_variables() {
   // Variables that don't depend on a database connection.
   $variables = array(
     'attributes' => array(),
-    'title_attributes' => array(),
-    'content_attributes' => array(),
     'title_prefix' => array(),
     'title_suffix' => array(),
     'db_is_active' => !defined('MAINTENANCE_MODE'),
@@ -1706,9 +1704,13 @@ function template_preprocess_field(&$variables, $hook) {
     // the <a> element of a field item being rendered as a link). Other field
     // formatters leave them within $element['#items'][$delta]['_attributes'] to
     // be rendered on the item wrappers provided by field.html.twig.
-    $variables['items'][$delta]['attributes'] = !empty($element['#items'][$delta]->_attributes) ? new Attribute($element['#items'][$delta]->_attributes) : clone($default_attributes);
-    $delta++;
+    foreach ($variables['items'] as $delta => $item) {
+      $variables['item_attributes'][$delta] = !empty($element['#items'][$delta]->_attributes) ? new Attribute($element['#items'][$delta]->_attributes) : clone $default_attributes;
+    }
   }
+
+  $variables['title_attributes'] = isset($variables['title_attributes']) ? new Attribute($variables['title_attributes']) : clone $default_attributes;
+  $variables['content_attributes'] = isset($variables['content_attributes']) ? new Attribute($variables['content_attributes']) : clone $default_attributes;
 }
 
 /**
diff --git a/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
index a100374..f86565b 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -345,15 +345,13 @@ protected function theme($hook, $variables = array()) {
       if (!isset($default_attributes)) {
         $default_attributes = new Attribute();
       }
-      foreach (array('attributes', 'title_attributes', 'content_attributes') as $key) {
-        if (isset($variables[$key]) && !($variables[$key] instanceof Attribute)) {
-          if ($variables[$key]) {
-            $variables[$key] = new Attribute($variables[$key]);
-          }
-          else {
-            // Create empty attributes.
-            $variables[$key] = clone $default_attributes;
-          }
+      if (isset($variables['attributes']) && !($variables['attributes'] instanceof Attribute)) {
+        if ($variables['attributes']) {
+          $variables['attributes'] = new Attribute($variables['attributes']);
+        }
+        else {
+          // Create empty attributes.
+          $variables['attributes'] = clone $default_attributes;
         }
       }
 
diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index 6506ab2..41bbef7 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -11,6 +11,7 @@
 use Drupal\language\ConfigurableLanguageInterface;
 use Drupal\system\Entity\Menu;
 use Drupal\block\Entity\Block;
+use Drupal\Core\Template\Attribute;
 
 /**
  * Implements hook_help().
@@ -257,6 +258,10 @@ function template_preprocess_block(&$variables) {
     $variables['attributes']['aria-describedby'] = $variables['title_attributes']['id'];
   }
 
+  $variables['title_attributes'] = isset($variables['title_attributes']) ? new Attribute($variables['title_attributes']) : new Attribute();
+
+  // Add default class for block content.
+  $variables['content_attributes'] = new Attribute($variables['content_attributes']);
 }
 
 /**
diff --git a/core/modules/comment/comment.module b/core/modules/comment/comment.module
index 0ee6f4c..c0dc016 100644
--- a/core/modules/comment/comment.module
+++ b/core/modules/comment/comment.module
@@ -24,6 +24,7 @@
 use Drupal\Core\Entity\Display\EntityViewDisplayInterface;
 use Drupal\Core\Field\FieldDefinitionInterface;
 use Drupal\Core\Render\Element;
+use Drupal\Core\Template\Attribute;
 use Drupal\Core\Url;
 use Drupal\field\Entity\FieldStorageConfig;
 use Drupal\field\FieldConfigInterface;
@@ -793,6 +794,9 @@ function template_preprocess_comment(&$variables) {
 
   // Add comment author user ID. Necessary for the comment-by-viewer library.
   $variables['attributes']['data-comment-user-id'] = $comment->getOwnerId();
+  $variables['content_attributes']['class'][] = 'content';
+  $variables['content_attributes'] = new Attribute($variables['content_attributes']);
+  $variables['title_attributes'] = isset($variables['title_attributes']) ? new Attribute($variables['title_attributes']) : new Attribute();
 }
 
 /**
@@ -817,7 +821,6 @@ function comment_preprocess_field(&$variables) {
 
     // Adjust a comment field's attributes.
     $variables['attributes']['class'][] = 'comment-wrapper';
-    $variables['title_attributes']['class'][] = 'title';
 
     // Append additional attributes (eg. RDFa) from the first field item.
     $variables['attributes'] += $variables['items'][0]['attributes']->storage();
@@ -825,7 +828,8 @@ function comment_preprocess_field(&$variables) {
     // Create separate variables for the comments and comment form.
     $variables['comments'] = $element[0]['comments'];
     $variables['comment_form'] = $element[0]['comment_form'];
-    $variables['content_attributes']['class'] = array('title', 'comment-form__title');
+    $variables['title_attributes']->addClass('title');
+    $variables['content_attributes']->addClass(array('title', 'comment-form__title'));
   }
 }
 
diff --git a/core/modules/search/search.pages.inc b/core/modules/search/search.pages.inc
index 4c84c84..2c45678 100644
--- a/core/modules/search/search.pages.inc
+++ b/core/modules/search/search.pages.inc
@@ -7,6 +7,7 @@
 
 use Drupal\Component\Utility\String;
 use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Template\Attribute;
 
 /**
  * Implements hook_theme_suggestions_HOOK().
@@ -60,5 +61,7 @@ function template_preprocess_search_result(&$variables) {
   // Provide separated and grouped meta information..
   $variables['info_split'] = $info;
   $variables['info'] = implode(' - ', $info);
+  $variables['title_attributes'] = isset($variables['title_attributes']) ? new Attribute($variables['title_attributes']) : new Attribute();
+  $variables['content_attributes'] = isset($variables['content_attributes']) ? new Attribute($variables['content_attributes']) : new Attribute();
 }
 
diff --git a/core/themes/bartik/bartik.theme b/core/themes/bartik/bartik.theme
index 3216ab2..f8e033a 100644
--- a/core/themes/bartik/bartik.theme
+++ b/core/themes/bartik/bartik.theme
@@ -133,9 +133,9 @@ function bartik_preprocess_menu(&$variables) {
 function bartik_preprocess_field(&$variables) {
   $element = $variables['element'];
   if ($element['#field_type'] == 'taxonomy_term_reference') {
-    $variables['title_attributes']['class'][] = 'field-label';
+    $variables['title_attributes']->addClass('field-label');
     if ($variables['element']['#label_display'] == 'inline') {
-      $variables['title_attributes']['class'][] = 'inline';
+      $variables['title_attributes']->addClass('inline');
     }
   }
 }
diff --git a/core/themes/bartik/templates/field--taxonomy-term-reference.html.twig b/core/themes/bartik/templates/field--taxonomy-term-reference.html.twig
index a8e94e2..8051999 100644
--- a/core/themes/bartik/templates/field--taxonomy-term-reference.html.twig
+++ b/core/themes/bartik/templates/field--taxonomy-term-reference.html.twig
@@ -22,8 +22,8 @@
     <h3{{ title_attributes }}>{{ label }}</h3>
   {% endif %}
   <ul{{ content_attributes.addClass('links', 'field-items') }}>
-    {% for item in items %}
-      <li{{ item.attributes }}>{{ item.content }}</li>
+    {% for delta, item in items %}
+      <li{{ item_attributes[delta].addClass('taxonomy-term-reference-' ~ delta) }}>{{ item }}</li>
     {% endfor %}
   </ul>
 </div>