diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index f7acb39..ef2e320 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -416,8 +416,6 @@ function list_themes($refresh = FALSE) {
  * @see template_preprocess()
  */
 function _theme($hook, $variables = array()) {
-  static $default_attributes;
-
   $module_handler = \Drupal::moduleHandler();
 
   // If called before all modules are loaded, we do not necessarily have a full
@@ -623,19 +621,8 @@ function _theme($hook, $variables = array()) {
       template_preprocess($default_template_variables, $hook, $info);
       $variables += $default_template_variables;
     }
-    if (!isset($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)) {
+      $variables['attributes'] = new Attribute($variables['attributes']);
     }
 
     // Render the output using the template file.
@@ -1088,8 +1075,6 @@ function template_preprocess_datetime(&$variables) {
 
   // Add a 'datetime' class.
   $variables['attributes']['class'][] = 'datetime';
-
-  $variables['attributes'] = new Attribute($variables['attributes']);
 }
 
 /**
@@ -1932,8 +1917,6 @@ function _template_preprocess_default_variables() {
   // Variables that don't depend on a database connection.
   $variables = array(
     'attributes' => array(),
-    'title_attributes' => array(),
-    'content_attributes' => array(),
     'title_prefix' => array(),
     'title_suffix' => array(),
     'db_is_active' => !defined('MAINTENANCE_MODE'),
@@ -2418,11 +2401,6 @@ function template_preprocess_field(&$variables, $hook) {
     $variables['title_attributes']['class'][] = 'visually-hidden';
   }
 
-  static $default_attributes;
-  if (!isset($default_attributes)) {
-    $default_attributes = new Attribute;
-  }
-
   // Modules (e.g., rdf.module) can add field item attributes (to
   // $item->_attributes) within hook_entity_prepare_view(). Some field
   // formatters move those attributes into some nested formatter-specific
@@ -2431,8 +2409,11 @@ function template_preprocess_field(&$variables, $hook) {
   // formatters leave them within $element['#items'][$delta]['_attributes'] to
   // be rendered on the item wrappers provided by field.html.twig.
   foreach ($variables['items'] as $delta => $item) {
-    $variables['item_attributes'][$delta] = !empty($element['#items'][$delta]->_attributes) ? new Attribute($element['#items'][$delta]->_attributes) : clone($default_attributes);
+    $variables['item_attributes'][$delta] = !empty($element['#items'][$delta]->_attributes) ? new Attribute($element['#items'][$delta]->_attributes) : new Attribute();
   }
+
+  $variables['title_attributes'] = isset($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/modules/block/block.module b/core/modules/block/block.module
index 78d9fad..96a075b 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -9,6 +9,7 @@
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\language\Entity\ConfigurableLanguage;
 use Drupal\system\Entity\Menu;
+use Drupal\Core\Template\Attribute;
 
 /**
  * Implements hook_help().
@@ -279,9 +280,6 @@ function template_preprocess_block(&$variables) {
   $variables['attributes']['class'][] = 'block';
   $variables['attributes']['class'][] = drupal_html_class('block-' . $variables['configuration']['provider']);
 
-  // Add default class for block content.
-  $variables['content_attributes']['class'][] = 'content';
-
   // Create a valid HTML ID and make sure it is unique.
   if (!empty($variables['elements']['#id'])) {
     $variables['attributes']['id'] = drupal_html_id('block-' . $variables['elements']['#id']);
@@ -293,6 +291,11 @@ function template_preprocess_block(&$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']['class'][] = 'content';
+  $variables['content_attributes'] = new Attribute($variables['content_attributes']);
 }
 
 /**
diff --git a/core/modules/block/src/Tests/BlockPreprocessUnitTest.php b/core/modules/block/src/Tests/BlockPreprocessUnitTest.php
index 9118e21..f98bbb3 100644
--- a/core/modules/block/src/Tests/BlockPreprocessUnitTest.php
+++ b/core/modules/block/src/Tests/BlockPreprocessUnitTest.php
@@ -50,7 +50,8 @@ function testBlockClasses() {
     // Test adding a class to the block content.
     $variables['content_attributes']['class'][] = 'test-class';
     template_preprocess_block($variables);
-    $this->assertEqual($variables['content_attributes']['class'], array('test-class', 'content'), 'Default .content class added to block content_attributes');
+    $classes = $variables['content_attributes']['class']->value();
+    $this->assertEqual($classes, array('test-class', 'content'), 'Default .content class added to block content_attributes');
   }
 
 }
diff --git a/core/modules/comment/comment.module b/core/modules/comment/comment.module
index 5ce3ba9..b6cd321 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\FieldInstanceConfigInterface;
@@ -988,6 +989,8 @@ function template_preprocess_comment(&$variables) {
   $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();
 }
 
 /**
@@ -1012,7 +1015,6 @@ 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'] += $variables['item_attributes'][0]->storage();
@@ -1020,7 +1022,8 @@ function comment_preprocess_field(&$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/node/node.module b/core/modules/node/node.module
index fefd6d5..cd4d58c 100644
--- a/core/modules/node/node.module
+++ b/core/modules/node/node.module
@@ -672,6 +672,8 @@ function template_preprocess_node(&$variables) {
   if (isset($variables['preview'])) {
     $variables['attributes']['class'][] = 'node--preview';
   }
+  $variables['title_attributes'] = isset($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/modules/search/search.module b/core/modules/search/search.module
index be8da8a..5c0e701 100644
--- a/core/modules/search/search.module
+++ b/core/modules/search/search.module
@@ -132,7 +132,7 @@ function search_permission() {
 function search_preprocess_block(&$variables) {
   if ($variables['plugin_id'] == 'search_form_block') {
     $variables['attributes']['role'] = 'search';
-    $variables['content_attributes']['class'][] = 'container-inline';
+    $variables['content_attributes']->addClass('container-inline');
   }
 }
 
diff --git a/core/modules/search/search.pages.inc b/core/modules/search/search.pages.inc
index 6590b90..13f688c 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 27f8973..e78d263 100644
--- a/core/themes/bartik/bartik.theme
+++ b/core/themes/bartik/bartik.theme
@@ -158,9 +158,9 @@ function bartik_menu_tree__shortcut_default($variables) {
 function bartik_preprocess_field(&$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');
     }
   }
 }