diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index f1a5f95..ab267da 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -1368,6 +1368,8 @@ abstract class WebTestBase extends TestBase {
             break;
           case 'restripe':
             break;
+          case 'add_css':
+            break;
         }
       }
       $content = $dom->saveHTML();
diff --git a/core/modules/system/lib/Drupal/system/Tests/Ajax/CommandsTest.php b/core/modules/system/lib/Drupal/system/Tests/Ajax/CommandsTest.php
index 299b674..faa35bb 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Ajax/CommandsTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Ajax/CommandsTest.php
@@ -157,5 +157,13 @@ class CommandsTest extends AjaxTestBase {
       'settings' => array('ajax_forms_test' => array('foo' => 42)),
     );
     $this->assertCommand($commands, $expected, "'settings' AJAX command issued with correct data");
+
+    // Tests the 'add_css' command.
+    $commands = $this->drupalPostAJAX($form_path, $edit, array('op' => t("AJAX 'add_css' command")));
+    $expected = array(
+      'command' => 'add_css',
+      'data' => 'my/file.css',
+    );
+    $this->assertCommand($commands, $expected, "'add_css' AJAX command issued with correct data");
   }
 }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsTest.php b/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsTest.php
index 441a2ba..397762b 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsTest.php
@@ -89,6 +89,10 @@ class CascadingStylesheetsTest extends WebTestBase {
     drupal_add_css($css);
     $styles = drupal_get_css();
     $this->assertTrue(strpos($styles, $css) > 0, t('Rendered CSS includes the added stylesheet.'));
+    // Verify that newlines are properly added inside style tags.
+    $query_string = variable_get('css_js_query_string', '0');
+    $css_processed = "<style media=\"all\">\n@import url(\"" . check_plain(file_create_url($css)) . "?" . $query_string ."\");\n</style>";
+    $this->assertEqual(trim($styles), $css_processed, t('Rendered CSS includes newlines inside style tags for JavaScript use.'));
   }
 
   /**
diff --git a/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module b/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module
index 6a95710..d4074d1 100644
--- a/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module
+++ b/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module
@@ -258,6 +258,15 @@ function ajax_forms_test_ajax_commands_form($form, &$form_state) {
     ),
   );
 
+  // Shows the Ajax 'add_css' command.
+  $form['add_css_command_example'] = array(
+    '#type' => 'submit',
+    '#value' => t("AJAX 'add_css' command"),
+    '#ajax' => array(
+      'callback' => 'ajax_forms_test_advanced_commands_add_css_callback',
+    ),
+  );
+
   $form['submit'] = array(
     '#type' => 'submit',
     '#value' => t('Submit'),
@@ -411,6 +420,15 @@ function ajax_forms_test_advanced_commands_settings_callback($form, $form_state)
 }
 
 /**
+ * Ajax callback for 'add_css'.
+ */
+function ajax_forms_test_advanced_commands_add_css_callback($form, $form_state) {
+  $commands = array();
+  $commands[] = ajax_command_add_css('my/file.css');
+  return array('#type' => 'ajax', '#commands' => $commands);
+}
+
+/**
  * This form and its related submit and callback functions demonstrate
  * not validating another form element when a single Ajax element is triggered.
  *