diff --git a/core/lib/Drupal/Core/Render/Element/StatusMessages.php b/core/lib/Drupal/Core/Render/Element/StatusMessages.php
index d16c245..530457f 100644
--- a/core/lib/Drupal/Core/Render/Element/StatusMessages.php
+++ b/core/lib/Drupal/Core/Render/Element/StatusMessages.php
@@ -73,17 +73,22 @@ public static function generatePlaceholder(array $element) {
    * @see drupal_get_messages()
    */
   public static function renderMessages($type) {
-    // Render the messages.
-    return [
-      '#theme' => 'status_messages',
-      // @todo Improve when https://www.drupal.org/node/2278383 lands.
-      '#message_list' => drupal_get_messages($type),
-      '#status_headings' => [
-        'status' => t('Status message'),
-        'error' => t('Error message'),
-        'warning' => t('Warning message'),
-      ],
-    ];
+    $render = [];
+    $messages = drupal_get_messages($type);
+    if ($messages) {
+      // Render the messages.
+      $render = [
+        '#theme' => 'status_messages',
+        // @todo Improve when https://www.drupal.org/node/2278383 lands.
+        '#message_list' => $messages,
+        '#status_headings' => [
+          'status' => t('Status message'),
+          'error' => t('Error message'),
+          'warning' => t('Warning message'),
+        ],
+      ];
+    }
+    return $render;
   }
 
 }
diff --git a/core/modules/big_pipe/src/Tests/BigPipePlaceholderTestCases.php b/core/modules/big_pipe/src/Tests/BigPipePlaceholderTestCases.php
index 4637780..7b5c33f 100644
--- a/core/modules/big_pipe/src/Tests/BigPipePlaceholderTestCases.php
+++ b/core/modules/big_pipe/src/Tests/BigPipePlaceholderTestCases.php
@@ -9,7 +9,6 @@
 
 use Drupal\big_pipe\Render\BigPipeMarkup;
 use Drupal\Core\Session\AccountInterface;
-use Drupal\Core\StringTranslation\PluralTranslatableMarkup;
 use Drupal\Core\Url;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
@@ -88,33 +87,14 @@ public static function cases(ContainerInterface $container = NULL, AccountInterf
     if ($container && $user) {
       $status_messages->embeddedAjaxResponseCommands = [
         [
-          'command' => 'settings',
-          'settings' => [
-            'ajaxPageState' => [
-              'theme' => 'classy',
-              'libraries' => 'big_pipe/big_pipe,classy/base,classy/messages,core/drupal.active-link,core/html5shiv,core/normalize,system/base',
-            ],
-            'pluralDelimiter' => PluralTranslatableMarkup::DELIMITER,
-            'user' => [
-              'uid' => '1',
-              'permissionsHash' => $container->get('user_permissions_hash_generator')->generate($user),
-            ],
-          ],
-          'merge' => TRUE,
-        ],
-        [
-          'command' => 'add_css',
-          'data' => '<link rel="stylesheet" href="' . base_path() . 'core/themes/classy/css/components/messages.css?' . $container->get('state')->get('system.css_js_query_string') . '" media="all" />' . "\n"
-        ],
-        [
           'command' => 'insert',
           'method' => 'replaceWith',
           'selector' => '[data-big-pipe-placeholder-id="callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&args%5B0%5D&token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA"]',
-          'data' => "\n" . '    <div role="contentinfo" aria-label="Status message" class="messages messages--status">' . "\n" . '                  <h2 class="visually-hidden">Status message</h2>' . "\n" . '                    Hello from BigPipe!' . "\n" . '            </div>' . "\n    ",
+          'data' => '    <div role="contentinfo" aria-label="Status message" class="messages messages--status">' . "\n" . '                  <h2 class="visually-hidden">Status message</h2>' . "\n" . '                    Hello from BigPipe!' . "\n" . '            </div>' . "\n    ",
           'settings' => NULL,
         ],
       ];
-      $status_messages->embeddedHtmlResponse = '<link rel="stylesheet" href="' . base_path() . 'core/themes/classy/css/components/messages.css?' . $container->get('state')->get('system.css_js_query_string') . '" media="all" />' . "\n" . "\n" . '    <div role="contentinfo" aria-label="Status message" class="messages messages--status">' . "\n" . '                  <h2 class="visually-hidden">Status message</h2>' . "\n" . '                    Hello from BigPipe!' . "\n" . '            </div>' . "\n    \n";
+      $status_messages->embeddedHtmlResponse = '<div role="contentinfo" aria-label="Status message" class="messages messages--status">' . "\n" . '                  <h2 class="visually-hidden">Status message</h2>' . "\n" . '                    Hello from BigPipe!' . "\n" . '            </div>' . "\n    \n";
     }
 
 
diff --git a/core/modules/system/system.post_update.php b/core/modules/system/system.post_update.php
index 5ed7ea0..36039df 100644
--- a/core/modules/system/system.post_update.php
+++ b/core/modules/system/system.post_update.php
@@ -65,6 +65,14 @@ function system_post_update_hashes_clear_cache() {
 function system_post_update_timestamp_plugins() {
   // Empty post-update hook.
 }
+
+/**
+ * Clear caches to ensure Classy's message library is always added.
+ */
+function system_post_update_classy_message_library() {
+  // Empty post-update hook.
+}
+
 /**
  * Force field type plugin definitions to be cleared.
  *
diff --git a/core/modules/system/tests/src/Functional/Theme/ThemeInfoTest.php b/core/modules/system/tests/src/Functional/Theme/ThemeInfoTest.php
index ee41968..534f614 100644
--- a/core/modules/system/tests/src/Functional/Theme/ThemeInfoTest.php
+++ b/core/modules/system/tests/src/Functional/Theme/ThemeInfoTest.php
@@ -90,13 +90,13 @@ public function testChanges() {
     $active_theme = $this->themeManager->getActiveTheme();
     // Make sure we are not testing the wrong theme.
     $this->assertEqual('test_theme', $active_theme->getName());
-    $this->assertEqual(['classy/base', 'core/normalize', 'test_theme/global-styling'], $active_theme->getLibraries());
+    $this->assertEqual(['classy/base', 'classy/messages', 'core/normalize', 'test_theme/global-styling'], $active_theme->getLibraries());
 
     // @see theme_test_system_info_alter()
     $this->state->set('theme_test.modify_info_files', TRUE);
     drupal_flush_all_caches();
     $active_theme = $this->themeManager->getActiveTheme();
-    $this->assertEqual(['classy/base', 'core/normalize', 'test_theme/global-styling', 'core/backbone'], $active_theme->getLibraries());
+    $this->assertEqual(['classy/base', 'classy/messages', 'core/normalize', 'test_theme/global-styling', 'core/backbone'], $active_theme->getLibraries());
   }
 
 }
diff --git a/core/tests/Drupal/FunctionalTests/Theme/BartikTest.php b/core/tests/Drupal/FunctionalTests/Theme/BartikTest.php
new file mode 100644
index 0000000..0ffd9af
--- /dev/null
+++ b/core/tests/Drupal/FunctionalTests/Theme/BartikTest.php
@@ -0,0 +1,42 @@
+<?php
+
+namespace Drupal\FunctionalTests\Theme;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests the Bartik theme.
+ *
+ * @group bartik
+ */
+class BartikTest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp() {
+    parent::setUp();
+
+    // Use the classy theme.
+    $this->assertTrue($this->container->get('theme_installer')->install(['bartik']));
+    $this->container->get('config.factory')
+      ->getEditable('system.theme')
+      ->set('default', 'bartik')
+      ->save();
+    // Clear the theme registry.
+    $this->container->set('theme.registry', NULL);
+  }
+
+  /**
+   * Tests the Bartik theme always adds classy's and its message CSS.
+   */
+  public function testRegressionMissingMessagesCss() {
+    $this->drupalGet('');
+    $this->assertSession()->statusCodeEquals(200);
+    // Ensure that Classy's message.css is loaded.
+    // @see classy.info.yml
+    $this->assertSession()->responseContains('classy/css/components/messages.css');
+    $this->assertSession()->responseContains('bartik/css/components/messages.css');
+  }
+
+}
diff --git a/core/tests/Drupal/FunctionalTests/Theme/ClassyTest.php b/core/tests/Drupal/FunctionalTests/Theme/ClassyTest.php
new file mode 100644
index 0000000..f4c87b4
--- /dev/null
+++ b/core/tests/Drupal/FunctionalTests/Theme/ClassyTest.php
@@ -0,0 +1,41 @@
+<?php
+
+namespace Drupal\FunctionalTests\Theme;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests the classy theme.
+ *
+ * @group classy
+ */
+class ClassyTest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp() {
+    parent::setUp();
+
+    // Use the classy theme.
+    $this->assertTrue($this->container->get('theme_installer')->install(['classy']));
+    $this->container->get('config.factory')
+      ->getEditable('system.theme')
+      ->set('default', 'classy')
+      ->save();
+    // Clear the theme registry.
+    $this->container->set('theme.registry', NULL);
+  }
+
+  /**
+   * Tests the Classy theme always adds it's message CSS.
+   */
+  public function testRegressionMissingMessagesCss() {
+    $this->drupalGet('');
+    $this->assertSession()->statusCodeEquals(200);
+    // Ensure that Classy's message.css is loaded.
+    // @see classy.info.yml
+    $this->assertSession()->responseContains('classy/css/components/messages.css');
+  }
+
+}
diff --git a/core/themes/classy/classy.info.yml b/core/themes/classy/classy.info.yml
index 05ef0d7..4685c3f 100644
--- a/core/themes/classy/classy.info.yml
+++ b/core/themes/classy/classy.info.yml
@@ -8,6 +8,7 @@ hidden: true
 
 libraries:
   - classy/base
+  - classy/messages
   - core/normalize
 
 libraries-extend:
diff --git a/core/themes/classy/templates/misc/status-messages.html.twig b/core/themes/classy/templates/misc/status-messages.html.twig
index bc8fd10..282df38 100644
--- a/core/themes/classy/templates/misc/status-messages.html.twig
+++ b/core/themes/classy/templates/misc/status-messages.html.twig
@@ -21,7 +21,6 @@
  *   - class: HTML classes.
  */
 #}
-{{ attach_library('classy/messages') }}
 {% block messages %}
 {% for type, messages in message_list %}
   {%