diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 5d645e2..340ac77 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1372,7 +1372,7 @@ function template_preprocess_page(&$variables) {
   $variables['language']          = $language_interface;
   $variables['logo']              = theme_get_setting('logo.url');
   $variables['site_name']         = (theme_get_setting('features.name') ? SafeMarkup::checkPlain($site_config->get('name')) : '');
-  $variables['site_slogan']       = (theme_get_setting('features.slogan') ? Xss::filterAdmin($site_config->get('slogan')) : '');
+  $variables['site_slogan']       = (theme_get_setting('features.slogan') ? SafeMarkup::checkAdminXss($site_config->get('slogan')) : '');
 
   // An exception might be thrown.
   try {
diff --git a/core/lib/Drupal/Component/Utility/SafeMarkup.php b/core/lib/Drupal/Component/Utility/SafeMarkup.php
index 69d12fe..b5d1d93 100644
--- a/core/lib/Drupal/Component/Utility/SafeMarkup.php
+++ b/core/lib/Drupal/Component/Utility/SafeMarkup.php
@@ -82,7 +82,7 @@ public static function set($string, $strategy = 'html') {
   /**
    * Checks if a string is safe to output.
    *
-   * @param string $string
+   * @param string|\Drupal\Component\Utility\SafeStringInterface $string
    *   The content to be checked.
    * @param string $strategy
    *   The escaping strategy. See self::set(). Defaults to 'html'.
@@ -91,7 +91,9 @@ public static function set($string, $strategy = 'html') {
    *   TRUE if the string has been marked secure, FALSE otherwise.
    */
   public static function isSafe($string, $strategy = 'html') {
-    return isset(static::$safeStrings[(string) $string][$strategy]) ||
+    // Do the instance of checks first to save unnecessarily casting the object
+    // to a string.
+    return $string instanceOf SafeStringInterface || isset(static::$safeStrings[(string) $string][$strategy]) ||
       isset(static::$safeStrings[(string) $string]['all']);
   }
 
@@ -149,7 +151,38 @@ public static function escape($string) {
    * @see \Drupal\Component\Utility\Xss::filterAdmin()
    */
   public static function checkAdminXss($string) {
-    return static::isSafe($string) ? $string : Xss::filterAdmin($string);
+    if (!static::isSafe($string)) {
+      $string = Xss::filterAdmin($string);
+      static::set($string);
+    }
+    return $string;
+  }
+
+  /**
+   * Filters HTML to prevent cross-site-scripting (XSS) vulnerabilities.
+   *
+   * This method is preferred to \Drupal\Component\Utility\Xss::filter() when
+   * the result is being used directly in the rendering system.
+   *
+   * @param $string
+   *   The string with raw HTML in it. It will be stripped of everything that
+   *   can cause an XSS attack.
+   * @param array $html_tags
+   *   An array of HTML tags.
+   *
+   * @return string
+   *   An XSS safe version of $string, or an empty string if $string is not
+   *   valid UTF-8. The string has been marked safe. If the string has already
+   *   been marked safe, it won't be escaped again.
+   *
+   * @see \Drupal\Component\Utility\Xss::filter()
+   */
+  public static function filterXss($string, $html_tags = array('a', 'em', 'strong', 'cite', 'blockquote', 'code', 'ul', 'ol', 'li', 'dl', 'dt', 'dd')) {
+    if (!static::isSafe($string)) {
+      $string = Xss::filter($string, $html_tags);
+      static::set($string);
+    }
+    return $string;
   }
 
   /**
@@ -209,10 +242,12 @@ public static function checkPlain($text) {
    *   any key in $args are replaced with the corresponding value, after
    *   optional sanitization and formatting. The type of sanitization and
    *   formatting depends on the first character of the key:
-   *   - @variable: Escaped to HTML using self::escape(). Use this as the
-   *     default choice for anything displayed on a page on the site.
-   *   - %variable: Escaped to HTML and formatted using self::placeholder(),
-   *     which makes the following HTML code:
+   *   - @variable: Escaped to plain text using htmlspecialchars() unless
+   *     self::isSafe() returns TRUE to indicate it was already escaped. Use
+   *     this as the default choice for anything displayed in HTML on the site.
+   *   - %variable: Escaped to plain text using htmlspecialchars() unless
+   *     self::isSafe() returns TRUE to indicate it was already escaped. It is
+   *     formatted with EM tags to yield HTML code like:
    *     @code
    *       <em class="placeholder">text output here.</em>
    *     @endcode
@@ -240,13 +275,13 @@ public static function format($string, array $args = array()) {
       switch ($key[0]) {
         case '@':
           // Escaped only.
-          $args[$key] = static::escape($value);
+          $args[$key] = static::isSafe($value) ? $value : htmlspecialchars($value, ENT_QUOTES, 'UTF-8');
           break;
 
         case '%':
         default:
           // Escaped and placeholder.
-          $args[$key] = static::placeholder($value);
+          $args[$key] = static::doPlaceholder($value);
           break;
 
         case '!':
@@ -268,21 +303,38 @@ public static function format($string, array $args = array()) {
   /**
    * Formats text for emphasized display in a placeholder inside a sentence.
    *
-   * Used automatically by self::format().
+   * The return value is flagged as a safe markup string for rendering.
    *
    * @param string $text
    *   The text to format (plain-text).
    *
    * @return string
-   *   The formatted text (html).
+   *   The escaped text formatted with EM tags to yield HTML code like:
+   *     @code
+   *       <em class="placeholder">text output here.</em>
+   *     @endcode
    */
   public static function placeholder($text) {
-    $string = '<em class="placeholder">' . static::escape($text) . '</em>';
+    $string = static::doPlaceholder($text);
     static::$safeStrings[$string]['html'] = TRUE;
     return $string;
   }
 
   /**
+   * Formats text for emphasized display in a placeholder inside a sentence.
+   *
+   * @param string $text
+   *   The text to format (plain-text).
+   *
+   * @return string
+   *   The formatted text (HTML) with wrapping EM tags.
+   */
+  protected static function doPlaceholder($text) {
+    $text = static::isSafe($text) ? $text : htmlspecialchars($text, ENT_QUOTES, 'UTF-8');
+    return '<em class="placeholder">' . $text . '</em>';
+  }
+
+  /**
    * Replaces all occurrences of the search string with the replacement string.
    *
    * Functions identically to str_replace(), but marks the returned output as
diff --git a/core/lib/Drupal/Component/Utility/SafeStringInterface.php b/core/lib/Drupal/Component/Utility/SafeStringInterface.php
new file mode 100644
index 0000000..1414a37
--- /dev/null
+++ b/core/lib/Drupal/Component/Utility/SafeStringInterface.php
@@ -0,0 +1,14 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Component\Utility\SafeStringInterface.
+ */
+
+namespace Drupal\Component\Utility;
+
+/**
+ * Marks an object's __toString() method as returning safe markup.
+ */
+interface SafeStringInterface {
+}
diff --git a/core/lib/Drupal/Component/Utility/UrlHelper.php b/core/lib/Drupal/Component/Utility/UrlHelper.php
index 6c87822..833e26b 100644
--- a/core/lib/Drupal/Component/Utility/UrlHelper.php
+++ b/core/lib/Drupal/Component/Utility/UrlHelper.php
@@ -271,8 +271,11 @@ public static function externalIsLocal($url, $base_url) {
   public static function filterBadProtocol($string) {
     // Get the plain text representation of the attribute value (i.e. its
     // meaning).
-    $string = Html::decodeEntities($string);
-    return SafeMarkup::checkPlain(static::stripDangerousProtocols($string));
+    $decoded_string = Html::decodeEntities($string);
+    if (strpos($decoded_string, ':')) {
+      $string = htmlspecialchars(static::stripDangerousProtocols($decoded_string), ENT_QUOTES, 'UTF-8');
+    }
+    return $string;
   }
 
   /**
diff --git a/core/lib/Drupal/Component/Utility/Xss.php b/core/lib/Drupal/Component/Utility/Xss.php
index 4c6e812..54269e3 100644
--- a/core/lib/Drupal/Component/Utility/Xss.php
+++ b/core/lib/Drupal/Component/Utility/Xss.php
@@ -29,14 +29,12 @@ class Xss {
    * Based on kses by Ulf Harnhammar, see http://sourceforge.net/projects/kses.
    * For examples of various XSS attacks, see: http://ha.ckers.org/xss.html.
    *
-   * This code does five things:
+   * This code does four things:
    * - Removes characters and constructs that can trick browsers.
    * - Makes sure all HTML entities are well-formed.
    * - Makes sure all HTML tags and attributes are well-formed.
    * - Makes sure no HTML tags contain URLs with a disallowed protocol (e.g.
    *   javascript:).
-   * - Marks the sanitized, XSS-safe version of $string as safe markup for
-   *   rendering.
    *
    * @param $string
    *   The string with raw HTML in it. It will be stripped of everything that
@@ -49,7 +47,7 @@ class Xss {
    *   valid UTF-8.
    *
    * @see \Drupal\Component\Utility\Unicode::validateUtf8()
-   * @see \Drupal\Component\Utility\SafeMarkup
+   * @see \Drupal\Component\Utility\SafeMarkup::filterXss
    *
    * @ingroup sanitization
    */
@@ -83,7 +81,7 @@ public static function filter($string, $html_tags = array('a', 'em', 'strong', '
     // for output. All other known XSS vectors have been filtered out by this
     // point and any HTML tags remaining will have been deliberately allowed, so
     // it is acceptable to call SafeMarkup::set() on the resultant string.
-    return SafeMarkup::set(preg_replace_callback('%
+    return preg_replace_callback('%
       (
       <(?=[^a-zA-Z!/])  # a lone <
       |                 # or
@@ -92,7 +90,7 @@ public static function filter($string, $html_tags = array('a', 'em', 'strong', '
       <[^>]*(>|$)       # a string that starts with a <, up until the > or the end of the string
       |                 # or
       >                 # just a >
-      )%x', $splitter, $string));
+      )%x', $splitter, $string);
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Ajax/AjaxResponse.php b/core/lib/Drupal/Core/Ajax/AjaxResponse.php
index a83898d..0898485 100644
--- a/core/lib/Drupal/Core/Ajax/AjaxResponse.php
+++ b/core/lib/Drupal/Core/Ajax/AjaxResponse.php
@@ -198,15 +198,15 @@ protected function ajaxRender(Request $request) {
     $renderer = $this->getRenderer();
     if (!empty($css_assets)) {
       $css_render_array = \Drupal::service('asset.css.collection_renderer')->render($css_assets);
-      $resource_commands[] = new AddCssCommand($renderer->render($css_render_array));
+      $resource_commands[] = new AddCssCommand((string) $renderer->render($css_render_array));
     }
     if (!empty($js_assets_header)) {
       $js_header_render_array = \Drupal::service('asset.js.collection_renderer')->render($js_assets_header);
-      $resource_commands[] = new PrependCommand('head', $renderer->render($js_header_render_array));
+      $resource_commands[] = new PrependCommand('head', (string) $renderer->render($js_header_render_array));
     }
     if (!empty($js_assets_footer)) {
       $js_footer_render_array = \Drupal::service('asset.js.collection_renderer')->render($js_assets_footer);
-      $resource_commands[] = new AppendCommand('body', $renderer->render($js_footer_render_array));
+      $resource_commands[] = new AppendCommand('body', (string) $renderer->render($js_footer_render_array));
     }
     foreach (array_reverse($resource_commands) as $resource_command) {
       $this->addCommand($resource_command, TRUE);
diff --git a/core/lib/Drupal/Core/Ajax/CommandWithAttachedAssetsTrait.php b/core/lib/Drupal/Core/Ajax/CommandWithAttachedAssetsTrait.php
index fe117e1..d83db57 100644
--- a/core/lib/Drupal/Core/Ajax/CommandWithAttachedAssetsTrait.php
+++ b/core/lib/Drupal/Core/Ajax/CommandWithAttachedAssetsTrait.php
@@ -37,10 +37,10 @@ protected function getRenderedContent() {
     if (is_array($this->content)) {
       $html = \Drupal::service('renderer')->renderRoot($this->content);
       $this->attachedAssets = AttachedAssets::createFromRenderArray($this->content);
-      return $html;
+      return (string) $html;
     }
     else {
-      return $this->content;
+      return (string) $this->content;
     }
   }
 
diff --git a/core/lib/Drupal/Core/Render/MainContent/AjaxRenderer.php b/core/lib/Drupal/Core/Render/MainContent/AjaxRenderer.php
index 31192f3..c54799d 100644
--- a/core/lib/Drupal/Core/Render/MainContent/AjaxRenderer.php
+++ b/core/lib/Drupal/Core/Render/MainContent/AjaxRenderer.php
@@ -64,7 +64,7 @@ public function renderResponse(array $main_content, Request $request, RouteMatch
       }
     }
 
-    $html = $this->drupalRenderRoot($main_content);
+    $html = (string) $this->drupalRenderRoot($main_content);
     $response->setAttachments($main_content['#attached']);
 
     // The selector for the insert command is NULL as the new content will
@@ -72,7 +72,7 @@ public function renderResponse(array $main_content, Request $request, RouteMatch
     // behavior can be changed with #ajax['method'].
     $response->addCommand(new InsertCommand(NULL, $html));
     $status_messages = array('#type' => 'status_messages');
-    $output = $this->drupalRenderRoot($status_messages);
+    $output = (string) $this->drupalRenderRoot($status_messages);
     if (!empty($output)) {
       $response->addCommand(new PrependCommand(NULL, $output));
     }
diff --git a/core/lib/Drupal/Core/Render/Renderer.php b/core/lib/Drupal/Core/Render/Renderer.php
index a91b14b..9125453 100644
--- a/core/lib/Drupal/Core/Render/Renderer.php
+++ b/core/lib/Drupal/Core/Render/Renderer.php
@@ -100,7 +100,8 @@ public function renderPlain(&$elements) {
     $this->resetStack();
     $output = $this->renderRoot($elements);
     static::$stack = $current_stack;
-    return $output;
+    // Convert any \Drupal\Core\Render\SafeString objects to a string.
+    return (string) $output;
   }
 
   /**
@@ -221,10 +222,10 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
         // to mark them as safe too. The parent markup contains the child
         // markup, so if the parent markup is safe, then the markup of the
         // individual children must be safe as well.
-        $elements['#markup'] = SafeMarkup::set($elements['#markup']);
+        $elements['#markup'] = new SafeString($elements['#markup']);
         if (!empty($elements['#cache_properties'])) {
           foreach (Element::children($cached_element) as $key) {
-            SafeMarkup::set($cached_element[$key]['#markup']);
+            $cached_element[$key]['#markup'] = new SafeString($cached_element[$key]['#markup']);
           }
         }
         // The render cache item contains all the bubbleable rendering metadata
@@ -392,7 +393,7 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
       foreach ($children as $key) {
         $elements['#children'] .= $this->doRender($elements[$key]);
       }
-      $elements['#children'] = SafeMarkup::set($elements['#children']);
+      $elements['#children'] = new SafeString($elements['#children'], 'UTF-8');
     }
 
     // If #theme is not implemented and the element has raw #markup as a
@@ -403,7 +404,7 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
     // required. Eventually #theme_wrappers will expect both #markup and
     // #children to be a single string as #children.
     if (!$theme_is_implemented && isset($elements['#markup'])) {
-      $elements['#children'] = SafeMarkup::set($elements['#markup'] . $elements['#children']);
+      $elements['#children'] = new SafeString($elements['#markup'] . $elements['#children']);
     }
 
     // Let the theme functions in #theme_wrappers add markup around the rendered
@@ -490,7 +491,7 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
     $this->bubbleStack();
 
     $elements['#printed'] = TRUE;
-    $elements['#markup'] = SafeMarkup::set($elements['#markup']);
+    $elements['#markup'] = new SafeString($elements['#markup']);
     return $elements['#markup'];
   }
 
diff --git a/core/lib/Drupal/Core/Render/RendererInterface.php b/core/lib/Drupal/Core/Render/RendererInterface.php
index 5d59d4d..bcee70e 100644
--- a/core/lib/Drupal/Core/Render/RendererInterface.php
+++ b/core/lib/Drupal/Core/Render/RendererInterface.php
@@ -25,7 +25,7 @@
    * @param array $elements
    *   The structured array describing the data to be rendered.
    *
-   * @return string
+   * @return \Drupal\Core\Render\SafeString
    *   The rendered HTML.
    *
    * @see ::render()
@@ -295,7 +295,7 @@ public function renderPlain(&$elements);
    *   (Internal use only.) Whether this is a recursive call or not. See
    *   ::renderRoot().
    *
-   * @return string
+   * @return \Drupal\Core\Render\SafeString
    *   The rendered HTML.
    *
    * @throws \LogicException
diff --git a/core/lib/Drupal/Core/Render/SafeString.php b/core/lib/Drupal/Core/Render/SafeString.php
new file mode 100644
index 0000000..d449284
--- /dev/null
+++ b/core/lib/Drupal/Core/Render/SafeString.php
@@ -0,0 +1,35 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Render\SafeString.
+ */
+
+namespace Drupal\Core\Render;
+
+use Drupal\Component\Utility\SafeStringInterface;
+use Drupal\Component\Utility\Unicode;
+
+/**
+ * Wrapper object to marks strings as safe during rendering.
+ *
+ * @see \Drupal\Core\Template\TwigExtension::escapeFilter
+ * @see \Twig_Markup
+ */
+class SafeString implements SafeStringInterface, \Countable {
+
+  protected $content;
+
+  public function __construct($content) {
+    $this->content = (string) $content;
+  }
+
+  public function __toString() {
+    return $this->content;
+  }
+
+  public function count() {
+    return Unicode::strlen($this->content);
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Template/Attribute.php b/core/lib/Drupal/Core/Template/Attribute.php
index ad4e1b1..b8c0590 100644
--- a/core/lib/Drupal/Core/Template/Attribute.php
+++ b/core/lib/Drupal/Core/Template/Attribute.php
@@ -7,7 +7,7 @@
 
 namespace Drupal\Core\Template;
 
-use Drupal\Component\Utility\SafeMarkup;
+use Drupal\Component\Utility\SafeStringInterface;
 
 /**
  * Collects, sanitizes, and renders HTML attributes.
@@ -42,8 +42,7 @@
  * The attribute keys and values are automatically sanitized for output with
  * htmlspecialchars() and the entire attribute string is marked safe for output.
  */
-class Attribute implements \ArrayAccess, \IteratorAggregate {
-
+class Attribute implements \ArrayAccess, \IteratorAggregate, SafeStringInterface {
   /**
    * Stores the attribute data.
    *
@@ -259,10 +258,7 @@ public function __toString() {
         $return .= ' ' . $rendered;
       }
     }
-    // The implementations of AttributeValueBase::render() call
-    // htmlspecialchars() on the attribute name and value so we are confident
-    // that the return value can be set as safe.
-    return SafeMarkup::set($return);
+    return $return;
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Template/TwigExtension.php b/core/lib/Drupal/Core/Template/TwigExtension.php
index 5a73640..4dfbdb6 100644
--- a/core/lib/Drupal/Core/Template/TwigExtension.php
+++ b/core/lib/Drupal/Core/Template/TwigExtension.php
@@ -13,6 +13,7 @@
 namespace Drupal\Core\Template;
 
 use Drupal\Component\Utility\SafeMarkup;
+use Drupal\Component\Utility\SafeStringInterface;
 use Drupal\Core\Render\RendererInterface;
 use Drupal\Core\Routing\UrlGeneratorInterface;
 use Drupal\Core\Theme\ThemeManagerInterface;
@@ -397,7 +398,7 @@ public function escapeFilter(\Twig_Environment $env, $arg, $strategy = 'html', $
     }
 
     // Keep Twig_Markup objects intact to support autoescaping.
-    if ($autoescape && $arg instanceOf \Twig_Markup) {
+    if ($autoescape && ($arg instanceOf \Twig_Markup || $arg instanceOf SafeStringInterface)) {
       return $arg;
     }
 
@@ -423,13 +424,16 @@ public function escapeFilter(\Twig_Environment $env, $arg, $strategy = 'html', $
 
     // We have a string or an object converted to a string: Autoescape it!
     if (isset($return)) {
-      if ($autoescape && SafeMarkup::isSafe($return, $strategy)) {
+      if (SafeMarkup::isSafe($return, $strategy)) {
         return $return;
       }
       // Drupal only supports the HTML escaping strategy, so provide a
       // fallback for other strategies.
       if ($strategy == 'html') {
-        return SafeMarkup::checkPlain($return);
+        // Call htmlspecialchars() directly as there is no point in marking
+        // something safe at this point.
+        // @see \Drupal\Component\Utility\SafeMarkup::checkPlain()
+        return htmlspecialchars($return, ENT_QUOTES, 'UTF-8');
       }
       return twig_escape_filter($env, $return, $strategy, $charset, $autoescape);
     }
diff --git a/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
index 6850fcb..caf9a90 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Core\Theme;
 
+use Drupal\Core\Render\SafeString;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Routing\StackedRouteMatchInterface;
 use Symfony\Component\HttpFoundation\RequestStack;
@@ -385,7 +386,7 @@ protected function theme($hook, $variables = array()) {
       $output = $render_function($template_file, $variables);
     }
 
-    return (string) $output;
+    return ($output instanceof SafeString) ? $output : (string) $output;
   }
 
   /**
diff --git a/core/modules/block/src/Tests/BlockViewBuilderTest.php b/core/modules/block/src/Tests/BlockViewBuilderTest.php
index 02fec0f..8ac1062 100644
--- a/core/modules/block/src/Tests/BlockViewBuilderTest.php
+++ b/core/modules/block/src/Tests/BlockViewBuilderTest.php
@@ -181,14 +181,14 @@ protected function verifyRenderCacheHandling() {
   public function testBlockViewBuilderAlter() {
     // Establish baseline.
     $build = $this->getBlockRenderArray();
-    $this->assertIdentical(drupal_render($build), 'Llamas &gt; unicorns!');
+    $this->assertIdentical((string) drupal_render($build), 'Llamas &gt; unicorns!');
 
     // Enable the block view alter hook that adds a suffix, for basic testing.
     \Drupal::state()->set('block_test_view_alter_suffix', TRUE);
     Cache::invalidateTags($this->block->getCacheTags());
     $build = $this->getBlockRenderArray();
     $this->assertTrue(isset($build['#suffix']) && $build['#suffix'] === '<br>Goodbye!', 'A block with content is altered.');
-    $this->assertIdentical(drupal_render($build), 'Llamas &gt; unicorns!<br>Goodbye!');
+    $this->assertIdentical((string) drupal_render($build), 'Llamas &gt; unicorns!<br>Goodbye!');
     \Drupal::state()->set('block_test_view_alter_suffix', FALSE);
 
     // Force a request via GET so we can get drupal_render() cache working.
@@ -209,7 +209,7 @@ public function testBlockViewBuilderAlter() {
     $expected_keys = array_merge($default_keys, array($alter_add_key));
     $build = $this->getBlockRenderArray();
     $this->assertIdentical($expected_keys, $build['#cache']['keys'], 'An altered cacheable block has the expected cache keys.');
-    $this->assertIdentical(drupal_render($build), '');
+    $this->assertIdentical((string) drupal_render($build), '');
     $cache_entry = $this->container->get('cache.render')->get($cid);
     $this->assertTrue($cache_entry, 'The block render element has been cached with the expected cache ID.');
     $expected_tags = array_merge($default_tags, ['rendered']);
@@ -224,7 +224,7 @@ public function testBlockViewBuilderAlter() {
     $build = $this->getBlockRenderArray();
     sort($build['#cache']['tags']);
     $this->assertIdentical($expected_tags, $build['#cache']['tags'], 'An altered cacheable block has the expected cache tags.');
-    $this->assertIdentical(drupal_render($build), '');
+    $this->assertIdentical((string) drupal_render($build), '');
     $cache_entry = $this->container->get('cache.render')->get($cid);
     $this->assertTrue($cache_entry, 'The block render element has been cached with the expected cache ID.');
     $expected_tags = array_merge($default_tags, [$alter_add_tag, 'rendered']);
@@ -237,7 +237,7 @@ public function testBlockViewBuilderAlter() {
     \Drupal::state()->set('block_test_view_alter_append_pre_render_prefix', TRUE);
     $build = $this->getBlockRenderArray();
     $this->assertFalse(isset($build['#prefix']), 'The appended #pre_render callback has not yet run before calling drupal_render().');
-    $this->assertIdentical(drupal_render($build), 'Hiya!<br>');
+    $this->assertIdentical((string) drupal_render($build), 'Hiya!<br>');
     $this->assertTrue(isset($build['#prefix']) && $build['#prefix'] === 'Hiya!<br>', 'A cached block without content is altered.');
 
     // Restore the previous request method.
diff --git a/core/modules/content_translation/src/ContentTranslationHandler.php b/core/modules/content_translation/src/ContentTranslationHandler.php
index 28529eb..7086cc8 100644
--- a/core/modules/content_translation/src/ContentTranslationHandler.php
+++ b/core/modules/content_translation/src/ContentTranslationHandler.php
@@ -290,8 +290,8 @@ public function entityFormAlter(array &$form, FormStateInterface $form_state, En
       $title = $this->entityFormTitle($entity);
       // When editing the original values display just the entity label.
       if ($form_langcode != $entity_langcode) {
-        $t_args = array('%language' => $languages[$form_langcode]->getName(), '%title' => $entity->label(), '!title' => $title);
-        $title = empty($source_langcode) ? t('!title [%language translation]', $t_args) : t('Create %language translation of %title', $t_args);
+        $t_args = array('%language' => $languages[$form_langcode]->getName(), '%title' => $entity->label(), '@title' => $title);
+        $title = empty($source_langcode) ? t('@title [%language translation]', $t_args) : t('Create %language translation of %title', $t_args);
       }
       $form['#title'] = $title;
     }
diff --git a/core/modules/contextual/src/ContextualController.php b/core/modules/contextual/src/ContextualController.php
index aeee4a6..dd2bed4 100644
--- a/core/modules/contextual/src/ContextualController.php
+++ b/core/modules/contextual/src/ContextualController.php
@@ -44,7 +44,7 @@ public function render(Request $request) {
         '#type' => 'contextual_links',
         '#contextual_links' => _contextual_id_to_links($id),
       );
-      $rendered[$id] = drupal_render($element);
+      $rendered[$id] = (string) drupal_render($element);
     }
 
     return new JsonResponse($rendered);
diff --git a/core/modules/editor/tests/src/Unit/EditorXssFilter/StandardTest.php b/core/modules/editor/tests/src/Unit/EditorXssFilter/StandardTest.php
index 99daa3b..c7f8e1d 100644
--- a/core/modules/editor/tests/src/Unit/EditorXssFilter/StandardTest.php
+++ b/core/modules/editor/tests/src/Unit/EditorXssFilter/StandardTest.php
@@ -105,7 +105,7 @@ public function providerTestFilterXss() {
 
     // Default SRC tag by leaving it empty.
     // @see https://www.owasp.org/index.php/XSS_Filter_Evasion_Cheat_Sheet#Default_SRC_tag_by_leaving_it_empty
-    $data[] = array('<IMG SRC= onmouseover="alert(\'xxs\')">', '<IMG nmouseover="alert(&#039;xxs&#039;)">');
+    $data[] = array('<IMG SRC= onmouseover="alert(\'xxs\')">', '<IMG nmouseover="alert(\'xxs\')">');
 
     // Default SRC tag by leaving it out entirely.
     // @see https://www.owasp.org/index.php/XSS_Filter_Evasion_Cheat_Sheet#Default_SRC_tag_by_leaving_it_out_entirely
diff --git a/core/modules/filter/filter.module b/core/modules/filter/filter.module
index 400d6cc..96d6aa3 100644
--- a/core/modules/filter/filter.module
+++ b/core/modules/filter/filter.module
@@ -430,7 +430,7 @@ function template_preprocess_filter_tips(&$variables) {
   foreach ($variables['tips'] as $name => $tiplist) {
     foreach ($tiplist as $tip_key => $tip) {
       $tiplist[$tip_key]['attributes'] = new Attribute();
-      $tiplist[$tip_key]['tip'] = Xss::filterAdmin($tiplist[$tip_key]['tip']);
+      $tiplist[$tip_key]['tip'] = SafeMarkup::checkAdminXss($tiplist[$tip_key]['tip']);
     }
 
     $variables['tips'][$name] = array(
diff --git a/core/modules/filter/src/Plugin/Filter/FilterCaption.php b/core/modules/filter/src/Plugin/Filter/FilterCaption.php
index 32977ec..acd4e09 100644
--- a/core/modules/filter/src/Plugin/Filter/FilterCaption.php
+++ b/core/modules/filter/src/Plugin/Filter/FilterCaption.php
@@ -45,7 +45,7 @@ public function process($text, $langcode) {
         // Sanitize caption: decode HTML encoding, limit allowed HTML tags; only
         // allow inline tags that are allowed by default, plus <br>.
         $caption = Html::decodeEntities($caption);
-        $caption = Xss::filter($caption, array('a', 'em', 'strong', 'cite', 'code', 'br'));
+        $caption = SafeMarkup::filterXss($caption, array('a', 'em', 'strong', 'cite', 'code', 'br'));
 
         // The caption must be non-empty.
         if (Unicode::strlen($caption) === 0) {
diff --git a/core/modules/quickedit/src/QuickEditController.php b/core/modules/quickedit/src/QuickEditController.php
index 383f61b..067a73a 100644
--- a/core/modules/quickedit/src/QuickEditController.php
+++ b/core/modules/quickedit/src/QuickEditController.php
@@ -216,7 +216,7 @@ public function fieldForm(EntityInterface $entity, $field_name, $langcode, $view
       $response->addCommand(new FieldFormSavedCommand($output, $other_view_modes));
     }
     else {
-      $output = $this->renderer->renderRoot($form);
+      $output = (string) $this->renderer->renderRoot($form);
       // When working with a hidden form, we don't want its CSS/JS to be loaded.
       if ($request->request->get('nocssjs') !== 'true') {
         $response->setAttachments($form['#attached']);
@@ -228,7 +228,7 @@ public function fieldForm(EntityInterface $entity, $field_name, $langcode, $view
         $status_messages = array(
           '#type' => 'status_messages'
         );
-        $response->addCommand(new FieldFormValidationErrorsCommand($this->renderer->renderRoot($status_messages)));
+        $response->addCommand(new FieldFormValidationErrorsCommand((string) $this->renderer->renderRoot($status_messages)));
       }
     }
 
@@ -275,7 +275,7 @@ protected function renderField(EntityInterface $entity, $field_name, $langcode,
       $output = $this->moduleHandler()->invoke($module, 'quickedit_render_field', $args);
     }
 
-    return $this->renderer->renderRoot($output);
+    return (string) $this->renderer->renderRoot($output);
   }
 
   /**
diff --git a/core/modules/rest/src/Tests/Views/StyleSerializerTest.php b/core/modules/rest/src/Tests/Views/StyleSerializerTest.php
index 1eca0d2..a0f482a 100644
--- a/core/modules/rest/src/Tests/Views/StyleSerializerTest.php
+++ b/core/modules/rest/src/Tests/Views/StyleSerializerTest.php
@@ -108,7 +108,7 @@ public function testSerializerResponses() {
     // Mock the request content type by setting it on the display handler.
     $view->display_handler->setContentType('json');
     $output = $view->preview();
-    $this->assertIdentical($actual_json, drupal_render_root($output), 'The expected JSON preview output was found.');
+    $this->assertIdentical($actual_json, (string) drupal_render_root($output), 'The expected JSON preview output was found.');
 
     // Test a 403 callback.
     $this->drupalGet('test/serialize/denied');
diff --git a/core/modules/search/search.module b/core/modules/search/search.module
index b4c4126..a888b1b 100644
--- a/core/modules/search/search.module
+++ b/core/modules/search/search.module
@@ -8,7 +8,6 @@
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Component\Utility\Html;
 use Drupal\Component\Utility\Unicode;
-use Drupal\Component\Utility\Xss;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Routing\RouteMatchInterface;
@@ -768,7 +767,7 @@ function search_excerpt($keys, $text, $langcode = NULL) {
   // Highlight keywords. Must be done at once to prevent conflicts ('strong'
   // and '<strong>').
   $text = trim(preg_replace('/' . $boundary . '(?:' . implode('|', $keys) . ')' . $boundary . '/iu', '<strong>\0</strong>', ' ' . $text . ' '));
-  return Xss::filter($text, ['strong']);
+  return SafeMarkup::filterXss($text, ['strong']);
 }
 
 /**
diff --git a/core/modules/simpletest/src/AssertContentTrait.php b/core/modules/simpletest/src/AssertContentTrait.php
index 240aea9..eb37d8e 100644
--- a/core/modules/simpletest/src/AssertContentTrait.php
+++ b/core/modules/simpletest/src/AssertContentTrait.php
@@ -397,7 +397,7 @@ protected function assertRaw($raw, $message = '', $group = 'Other') {
     if (!$message) {
       $message = SafeMarkup::format('Raw "@raw" found', array('@raw' => $raw));
     }
-    return $this->assert(strpos($this->getRawContent(), $raw) !== FALSE, $message, $group);
+    return $this->assert(strpos($this->getRawContent(), (string)  $raw) !== FALSE, $message, $group);
   }
 
   /**
@@ -424,7 +424,7 @@ protected function assertNoRaw($raw, $message = '', $group = 'Other') {
     if (!$message) {
       $message = SafeMarkup::format('Raw "@raw" not found', array('@raw' => $raw));
     }
-    return $this->assert(strpos($this->getRawContent(), $raw) === FALSE, $message, $group);
+    return $this->assert(strpos($this->getRawContent(), (string) $raw) === FALSE, $message, $group);
   }
 
   /**
@@ -808,7 +808,7 @@ protected function assertNoTitle($title, $message = '', $group = 'Other') {
    *   TRUE on pass, FALSE on fail.
    */
   protected function assertThemeOutput($callback, array $variables = array(), $expected = '', $message = '', $group = 'Other') {
-    $output = \Drupal::theme()->render($callback, $variables);
+    $output = (string) \Drupal::theme()->render($callback, $variables);
     $this->verbose(
       '<hr />' . 'Result:' . '<pre>' . SafeMarkup::checkPlain(var_export($output, TRUE)) . '</pre>'
       . '<hr />' . 'Expected:' . '<pre>' . SafeMarkup::checkPlain(var_export($expected, TRUE)) . '</pre>'
diff --git a/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php b/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php
index c5ebaa5..f619bd1 100644
--- a/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\system\Plugin\Block;
 
+use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Core\Block\BlockBase;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Config\ConfigFactoryInterface;
@@ -173,7 +174,7 @@ public function build() {
     );
 
     $build['site_slogan'] = array(
-      '#markup' => Xss::filterAdmin($site_config->get('slogan')),
+      '#markup' => SafeMarkup::checkAdminXss($site_config->get('slogan')),
       '#access' => $this->configuration['use_site_slogan'],
     );
 
diff --git a/core/modules/system/src/Tests/Common/RenderElementTypesTest.php b/core/modules/system/src/Tests/Common/RenderElementTypesTest.php
index 3aa3cec..822e77e 100644
--- a/core/modules/system/src/Tests/Common/RenderElementTypesTest.php
+++ b/core/modules/system/src/Tests/Common/RenderElementTypesTest.php
@@ -43,7 +43,7 @@ protected function setUp() {
    *   Assertion message.
    */
   protected function assertElements(array $elements, $expected_html, $message) {
-    $actual_html = drupal_render($elements);
+    $actual_html = (string) drupal_render($elements);
 
     $out = '<table><tr>';
     $out .= '<td valign="top"><pre>' . SafeMarkup::checkPlain($expected_html) . '</pre></td>';
diff --git a/core/modules/system/src/Tests/Entity/EntityTranslationTest.php b/core/modules/system/src/Tests/Entity/EntityTranslationTest.php
index 7d513eb..e8fe40d 100644
--- a/core/modules/system/src/Tests/Entity/EntityTranslationTest.php
+++ b/core/modules/system/src/Tests/Entity/EntityTranslationTest.php
@@ -609,10 +609,10 @@ protected function doTestLanguageFallback($entity_type) {
     // Get an view builder.
     $controller = $this->entityManager->getViewBuilder($entity_type);
     $entity2_build = $controller->view($entity2);
-    $entity2_output = drupal_render($entity2_build);
+    $entity2_output = (string) drupal_render($entity2_build);
     $translation = $this->entityManager->getTranslationFromContext($entity2, $default_langcode);
     $translation_build = $controller->view($translation);
-    $translation_output = drupal_render($translation_build);
+    $translation_output = (string) drupal_render($translation_build);
     $this->assertIdentical($entity2_output, $translation_output, 'When the entity has no translation no fallback is applied.');
 
     // Checks that entity translations are rendered properly.
diff --git a/core/modules/system/templates/dropbutton-wrapper.html.twig b/core/modules/system/templates/dropbutton-wrapper.html.twig
index ca0ff7e..d4c8d90 100644
--- a/core/modules/system/templates/dropbutton-wrapper.html.twig
+++ b/core/modules/system/templates/dropbutton-wrapper.html.twig
@@ -12,7 +12,7 @@
  * @ingroup themeable
  */
 #}
-{% if children %}
+{% if children|length %}
   {% spaceless %}
     <div class="dropbutton-wrapper">
       <div class="dropbutton-widget">
diff --git a/core/modules/views/src/Plugin/views/PluginBase.php b/core/modules/views/src/Plugin/views/PluginBase.php
index 8c1e31f..a11e159 100644
--- a/core/modules/views/src/Plugin/views/PluginBase.php
+++ b/core/modules/views/src/Plugin/views/PluginBase.php
@@ -377,7 +377,7 @@ protected function viewsTokenReplace($text, $tokens) {
         '#context' => $twig_tokens,
       );
 
-      return $this->getRenderer()->render($build);
+      return (string) $this->getRenderer()->render($build);
     }
     else {
       return $text;
diff --git a/core/modules/views/src/Plugin/views/display/Feed.php b/core/modules/views/src/Plugin/views/display/Feed.php
index 0798690..e4da10e 100644
--- a/core/modules/views/src/Plugin/views/display/Feed.php
+++ b/core/modules/views/src/Plugin/views/display/Feed.php
@@ -67,7 +67,7 @@ public static function buildResponse($view_id, $display_id, array $args = []) {
     /** @var \Drupal\Core\Render\RendererInterface $renderer */
     $renderer = \Drupal::service('renderer');
 
-    $output = $renderer->renderRoot($build);
+    $output = (string) $renderer->renderRoot($build);
 
     if (empty($output)) {
       throw new NotFoundHttpException();
diff --git a/core/modules/views/src/Plugin/views/field/FieldPluginBase.php b/core/modules/views/src/Plugin/views/field/FieldPluginBase.php
index f17df18..415831f 100644
--- a/core/modules/views/src/Plugin/views/field/FieldPluginBase.php
+++ b/core/modules/views/src/Plugin/views/field/FieldPluginBase.php
@@ -1131,7 +1131,7 @@ public function advancedRender(ResultRow $values) {
     else {
       $value = $this->render($values);
       if (is_array($value)) {
-        $value = $this->getRenderer()->render($value);
+        $value = (string) $this->getRenderer()->render($value);
       }
       $this->last_render = $value;
       $this->original_value = $value;
@@ -1144,7 +1144,7 @@ public function advancedRender(ResultRow $values) {
         foreach ($raw_items as $count => $item) {
           $value = $this->render_item($count, $item);
           if (is_array($value)) {
-            $value = $this->getRenderer()->render($value);
+            $value = (string) $this->getRenderer()->render($value);
           }
           $this->last_render = $value;
           $this->original_value = $this->last_render;
@@ -1162,7 +1162,7 @@ public function advancedRender(ResultRow $values) {
       }
 
       if (is_array($value)) {
-        $value = $this->getRenderer()->render($value);
+        $value = (string) $this->getRenderer()->render($value);
       }
       // This happens here so that renderAsLink can get the unaltered value of
       // this field as a token rather than the altered value.
diff --git a/core/modules/views/src/Plugin/views/row/OpmlFields.php b/core/modules/views/src/Plugin/views/row/OpmlFields.php
index 2e936f5..4654bca 100644
--- a/core/modules/views/src/Plugin/views/row/OpmlFields.php
+++ b/core/modules/views/src/Plugin/views/row/OpmlFields.php
@@ -217,7 +217,7 @@ public function getField($index, $field_id) {
     if (empty($this->view->style_plugin) || !is_object($this->view->style_plugin) || empty($field_id)) {
       return '';
     }
-    return $this->view->style_plugin->getField($index, $field_id);
+    return (string) $this->view->style_plugin->getField($index, $field_id);
   }
 
 }
diff --git a/core/modules/views/src/Tests/Handler/FieldFieldTest.php b/core/modules/views/src/Tests/Handler/FieldFieldTest.php
index fd0da52..fb44dc6 100644
--- a/core/modules/views/src/Tests/Handler/FieldFieldTest.php
+++ b/core/modules/views/src/Tests/Handler/FieldFieldTest.php
@@ -251,18 +251,18 @@ public function testSimpleRender() {
     $executable = Views::getView('test_field_field_test');
     $executable->execute();
 
-    $this->assertEqual(1, $executable->getStyle()->getField(0, 'id'));
-    $this->assertEqual(3, $executable->getStyle()->getField(0, 'field_test'));
-    $this->assertEqual(2, $executable->getStyle()->getField(1, 'id'));
+    $this->assertEqual(1, (string) $executable->getStyle()->getField(0, 'id'));
+    $this->assertEqual(3, (string) $executable->getStyle()->getField(0, 'field_test'));
+    $this->assertEqual(2, (string) $executable->getStyle()->getField(1, 'id'));
     // @todo Switch this assertion to assertIdentical('', ...) when
     //   https://www.drupal.org/node/2488006 gets fixed.
-    $this->assertEqual(0, $executable->getStyle()->getField(1, 'field_test'));
-    $this->assertEqual(3, $executable->getStyle()->getField(2, 'id'));
-    $this->assertEqual(8, $executable->getStyle()->getField(2, 'field_test'));
-    $this->assertEqual(4, $executable->getStyle()->getField(3, 'id'));
-    $this->assertEqual(5, $executable->getStyle()->getField(3, 'field_test'));
-    $this->assertEqual(5, $executable->getStyle()->getField(4, 'id'));
-    $this->assertEqual(6, $executable->getStyle()->getField(4, 'field_test'));
+    $this->assertEqual(0, (string) $executable->getStyle()->getField(1, 'field_test'));
+    $this->assertEqual(3, (string) $executable->getStyle()->getField(2, 'id'));
+    $this->assertEqual(8, (string) $executable->getStyle()->getField(2, 'field_test'));
+    $this->assertEqual(4, (string) $executable->getStyle()->getField(3, 'id'));
+    $this->assertEqual(5, (string) $executable->getStyle()->getField(3, 'field_test'));
+    $this->assertEqual(5, (string) $executable->getStyle()->getField(4, 'id'));
+    $this->assertEqual(6, (string) $executable->getStyle()->getField(4, 'field_test'));
   }
 
   /**
@@ -326,10 +326,10 @@ public function testFieldAliasRender() {
     $executable->execute();
 
     for ($i = 0; $i < 5; $i++) {
-      $this->assertEqual($i + 1, $executable->getStyle()->getField($i, 'id'));
-      $this->assertEqual('test ' . $i, $executable->getStyle()->getField($i, 'name'));
+      $this->assertEqual($i + 1, (string) $executable->getStyle()->getField($i, 'id'));
+      $this->assertEqual('test ' . $i, (string) $executable->getStyle()->getField($i, 'name'));
       $entity = EntityTest::load($i + 1);
-      $this->assertEqual('<a href="' . $entity->url() . '" hreflang="' . $entity->language()->getId() . '">test ' . $i . '</a>', $executable->getStyle()->getField($i, 'name_alias'));
+      $this->assertEqual('<a href="' . $entity->url() . '" hreflang="' . $entity->language()->getId() . '">test ' . $i . '</a>', (string) $executable->getStyle()->getField($i, 'name_alias'));
     }
   }
 
@@ -426,25 +426,25 @@ public function testRevisionRender() {
     $executable = Views::getView('test_field_field_revision_test');
     $executable->execute();
 
-    $this->assertEqual(1, $executable->getStyle()->getField(0, 'id'));
-    $this->assertEqual(1, $executable->getStyle()->getField(0, 'revision_id'));
-    $this->assertEqual(1, $executable->getStyle()->getField(0, 'field_test'));
-    $this->assertEqual('base value', $executable->getStyle()->getField(0, 'name'));
-
-    $this->assertEqual(1, $executable->getStyle()->getField(1, 'id'));
-    $this->assertEqual(2, $executable->getStyle()->getField(1, 'revision_id'));
-    $this->assertEqual(2, $executable->getStyle()->getField(1, 'field_test'));
-    $this->assertEqual('revision value1', $executable->getStyle()->getField(1, 'name'));
-
-    $this->assertEqual(1, $executable->getStyle()->getField(2, 'id'));
-    $this->assertEqual(3, $executable->getStyle()->getField(2, 'revision_id'));
-    $this->assertEqual(3, $executable->getStyle()->getField(2, 'field_test'));
-    $this->assertEqual('revision value2', $executable->getStyle()->getField(2, 'name'));
-
-    $this->assertEqual(2, $executable->getStyle()->getField(3, 'id'));
-    $this->assertEqual(4, $executable->getStyle()->getField(3, 'revision_id'));
-    $this->assertEqual(4, $executable->getStyle()->getField(3, 'field_test'));
-    $this->assertEqual('next entity value', $executable->getStyle()->getField(3, 'name'));
+    $this->assertEqual(1, (string) $executable->getStyle()->getField(0, 'id'));
+    $this->assertEqual(1, (string) $executable->getStyle()->getField(0, 'revision_id'));
+    $this->assertEqual(1, (string) $executable->getStyle()->getField(0, 'field_test'));
+    $this->assertEqual('base value', (string) $executable->getStyle()->getField(0, 'name'));
+
+    $this->assertEqual(1, (string) $executable->getStyle()->getField(1, 'id'));
+    $this->assertEqual(2, (string) $executable->getStyle()->getField(1, 'revision_id'));
+    $this->assertEqual(2, (string) $executable->getStyle()->getField(1, 'field_test'));
+    $this->assertEqual('revision value1', (string) $executable->getStyle()->getField(1, 'name'));
+
+    $this->assertEqual(1, (string) $executable->getStyle()->getField(2, 'id'));
+    $this->assertEqual(3, (string) $executable->getStyle()->getField(2, 'revision_id'));
+    $this->assertEqual(3, (string) $executable->getStyle()->getField(2, 'field_test'));
+    $this->assertEqual('revision value2', (string) $executable->getStyle()->getField(2, 'name'));
+
+    $this->assertEqual(2, (string) $executable->getStyle()->getField(3, 'id'));
+    $this->assertEqual(4, (string) $executable->getStyle()->getField(3, 'revision_id'));
+    $this->assertEqual(4, (string) $executable->getStyle()->getField(3, 'field_test'));
+    $this->assertEqual('next entity value', (string) $executable->getStyle()->getField(3, 'name'));
   }
 
   /**
@@ -484,33 +484,33 @@ public function testRevisionComplexRender() {
     $executable = Views::getView('test_field_field_revision_complex_test');
     $executable->execute();
 
-    $this->assertEqual(1, $executable->getStyle()->getField(0, 'id'));
-    $this->assertEqual(1, $executable->getStyle()->getField(0, 'revision_id'));
-    $this->assertEqual($this->testUsers[0]->getTimeZone(), $executable->getStyle()->getField(0, 'timezone'));
-    $this->assertEqual('1, 3, 7', $executable->getStyle()->getField(0, 'field_test_multiple'));
-    $this->assertEqual('1', $executable->getStyle()->getField(0, 'field_test_multiple_1'));
-    $this->assertEqual('3, 7', $executable->getStyle()->getField(0, 'field_test_multiple_2'));
-
-    $this->assertEqual(1, $executable->getStyle()->getField(1, 'id'));
-    $this->assertEqual(2, $executable->getStyle()->getField(1, 'revision_id'));
-    $this->assertEqual($this->testUsers[1]->getTimeZone(), $executable->getStyle()->getField(1, 'timezone'));
-    $this->assertEqual('0, 3, 5', $executable->getStyle()->getField(1, 'field_test_multiple'));
-    $this->assertEqual('0', $executable->getStyle()->getField(1, 'field_test_multiple_1'));
-    $this->assertEqual('3, 5', $executable->getStyle()->getField(1, 'field_test_multiple_2'));
-
-    $this->assertEqual(1, $executable->getStyle()->getField(2, 'id'));
-    $this->assertEqual(3, $executable->getStyle()->getField(2, 'revision_id'));
-    $this->assertEqual($this->testUsers[2]->getTimeZone(), $executable->getStyle()->getField(2, 'timezone'));
-    $this->assertEqual('9, 9, 9', $executable->getStyle()->getField(2, 'field_test_multiple'));
-    $this->assertEqual('9', $executable->getStyle()->getField(2, 'field_test_multiple_1'));
-    $this->assertEqual('9, 9', $executable->getStyle()->getField(2, 'field_test_multiple_2'));
-
-    $this->assertEqual(2, $executable->getStyle()->getField(3, 'id'));
-    $this->assertEqual(4, $executable->getStyle()->getField(3, 'revision_id'));
-    $this->assertEqual($this->testUsers[3]->getTimeZone(), $executable->getStyle()->getField(3, 'timezone'));
-    $this->assertEqual('2, 9, 9', $executable->getStyle()->getField(3, 'field_test_multiple'));
-    $this->assertEqual('2', $executable->getStyle()->getField(3, 'field_test_multiple_1'));
-    $this->assertEqual('9, 9', $executable->getStyle()->getField(3, 'field_test_multiple_2'));
+    $this->assertEqual(1, (string) $executable->getStyle()->getField(0, 'id'));
+    $this->assertEqual(1, (string) $executable->getStyle()->getField(0, 'revision_id'));
+    $this->assertEqual($this->testUsers[0]->getTimeZone(), (string) $executable->getStyle()->getField(0, 'timezone'));
+    $this->assertEqual('1, 3, 7', (string) $executable->getStyle()->getField(0, 'field_test_multiple'));
+    $this->assertEqual('1', (string) $executable->getStyle()->getField(0, 'field_test_multiple_1'));
+    $this->assertEqual('3, 7', (string) $executable->getStyle()->getField(0, 'field_test_multiple_2'));
+
+    $this->assertEqual(1, (string) $executable->getStyle()->getField(1, 'id'));
+    $this->assertEqual(2, (string) $executable->getStyle()->getField(1, 'revision_id'));
+    $this->assertEqual($this->testUsers[1]->getTimeZone(), (string) $executable->getStyle()->getField(1, 'timezone'));
+    $this->assertEqual('0, 3, 5', (string) $executable->getStyle()->getField(1, 'field_test_multiple'));
+    $this->assertEqual('0', (string) $executable->getStyle()->getField(1, 'field_test_multiple_1'));
+    $this->assertEqual('3, 5', (string) $executable->getStyle()->getField(1, 'field_test_multiple_2'));
+
+    $this->assertEqual(1, (string) $executable->getStyle()->getField(2, 'id'));
+    $this->assertEqual(3, (string) $executable->getStyle()->getField(2, 'revision_id'));
+    $this->assertEqual($this->testUsers[2]->getTimeZone(), (string) $executable->getStyle()->getField(2, 'timezone'));
+    $this->assertEqual('9, 9, 9', (string) $executable->getStyle()->getField(2, 'field_test_multiple'));
+    $this->assertEqual('9', (string) $executable->getStyle()->getField(2, 'field_test_multiple_1'));
+    $this->assertEqual('9, 9', (string) $executable->getStyle()->getField(2, 'field_test_multiple_2'));
+
+    $this->assertEqual(2, (string) $executable->getStyle()->getField(3, 'id'));
+    $this->assertEqual(4, (string) $executable->getStyle()->getField(3, 'revision_id'));
+    $this->assertEqual($this->testUsers[3]->getTimeZone(), (string) $executable->getStyle()->getField(3, 'timezone'));
+    $this->assertEqual('2, 9, 9', (string) $executable->getStyle()->getField(3, 'field_test_multiple'));
+    $this->assertEqual('2', (string) $executable->getStyle()->getField(3, 'field_test_multiple_1'));
+    $this->assertEqual('9, 9', (string) $executable->getStyle()->getField(3, 'field_test_multiple_2'));
   }
 
   /**
@@ -533,7 +533,7 @@ public function testMissingBundleFieldRender() {
 
     // @todo Switch this assertion to assertIdentical('', ...) when
     //   https://www.drupal.org/node/2488006 gets fixed.
-    $this->assertEqual(0, $executable->getStyle()->getField(1, 'field_test'));
+    $this->assertEqual(0, (string) $executable->getStyle()->getField(1, 'field_test'));
   }
 
 }
diff --git a/core/modules/views/src/Tests/QueryGroupByTest.php b/core/modules/views/src/Tests/QueryGroupByTest.php
index 768aaaf..ad6e28e 100644
--- a/core/modules/views/src/Tests/QueryGroupByTest.php
+++ b/core/modules/views/src/Tests/QueryGroupByTest.php
@@ -253,9 +253,9 @@ public function testGroupByFieldWithCardinality() {
     $this->executeView($view);
     $this->assertEqual(2, count($view->result));
 
-    $this->assertEqual(3, $view->getStyle()->getField(0, 'id'));
+    $this->assertEqual(3, (string) $view->getStyle()->getField(0, 'id'));
     $this->assertEqual('1', $view->getStyle()->getField(0, 'field_test'));
-    $this->assertEqual(6, $view->getStyle()->getField(1, 'id'));
+    $this->assertEqual(6, (string) $view->getStyle()->getField(1, 'id'));
     $this->assertEqual('2', $view->getStyle()->getField(1, 'field_test'));
 
     $entities[2]->field_test[0]->value = 3;
@@ -267,15 +267,15 @@ public function testGroupByFieldWithCardinality() {
     $this->executeView($view);
     $this->assertEqual(5, count($view->result));
 
-    $this->assertEqual(3, $view->getStyle()->getField(0, 'id'));
+    $this->assertEqual(3, (string) $view->getStyle()->getField(0, 'id'));
     $this->assertEqual('1', $view->getStyle()->getField(0, 'field_test'));
-    $this->assertEqual(3, $view->getStyle()->getField(1, 'id'));
+    $this->assertEqual(3, (string) $view->getStyle()->getField(1, 'id'));
     $this->assertEqual('2', $view->getStyle()->getField(1, 'field_test'));
-    $this->assertEqual(1, $view->getStyle()->getField(2, 'id'));
+    $this->assertEqual(1, (string) $view->getStyle()->getField(2, 'id'));
     $this->assertEqual('3', $view->getStyle()->getField(2, 'field_test'));
-    $this->assertEqual(1, $view->getStyle()->getField(3, 'id'));
+    $this->assertEqual(1, (string) $view->getStyle()->getField(3, 'id'));
     $this->assertEqual('4', $view->getStyle()->getField(3, 'field_test'));
-    $this->assertEqual(1, $view->getStyle()->getField(4, 'id'));
+    $this->assertEqual(1, (string) $view->getStyle()->getField(4, 'id'));
     $this->assertEqual('5', $view->getStyle()->getField(4, 'field_test'));
 
     // Check that translated values are correctly retrieved and are not grouped
@@ -288,7 +288,7 @@ public function testGroupByFieldWithCardinality() {
     $this->executeView($view);
 
     $this->assertEqual(6, count($view->result));
-    $this->assertEqual(3, $view->getStyle()->getField(5, 'id'));
+    $this->assertEqual(3, (string) $view->getStyle()->getField(5, 'id'));
     $this->assertEqual('6', $view->getStyle()->getField(5, 'field_test'));
   }
 
diff --git a/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php b/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php
index a3fe908..4472bcd 100644
--- a/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php
@@ -237,8 +237,8 @@ public function testUncacheableParent($element, $args, $placeholder_cid_keys, ar
     // No #cache on parent element.
     $element['#prefix'] = '<p>#cache disabled</p>';
     $output = $this->renderer->renderRoot($element);
-    $this->assertSame('<p>#cache disabled</p><p>This is a rendered placeholder!</p>', $output, 'Output is overridden.');
-    $this->assertSame('<p>#cache disabled</p><p>This is a rendered placeholder!</p>', $element['#markup'], '#markup is overridden.');
+    $this->assertSame('<p>#cache disabled</p><p>This is a rendered placeholder!</p>', (string) $output, 'Output is overridden.');
+    $this->assertSame('<p>#cache disabled</p><p>This is a rendered placeholder!</p>', (string) $element['#markup'], '#markup is overridden.');
     $expected_js_settings = [
       'foo' => 'bar',
       'dynamic_animal' => $args[0],
@@ -283,9 +283,9 @@ public function testCacheableParent($test_element, $args, $placeholder_cid_keys,
     $element['#cache'] = ['keys' => ['placeholder_test_GET']];
     $element['#prefix'] = '<p>#cache enabled, GET</p>';
     $output = $this->renderer->renderRoot($element);
-    $this->assertSame('<p>#cache enabled, GET</p><p>This is a rendered placeholder!</p>', $output, 'Output is overridden.');
+    $this->assertSame('<p>#cache enabled, GET</p><p>This is a rendered placeholder!</p>', (string) $output, 'Output is overridden.');
     $this->assertTrue(isset($element['#printed']), 'No cache hit');
-    $this->assertSame('<p>#cache enabled, GET</p><p>This is a rendered placeholder!</p>', $element['#markup'], '#markup is overridden.');
+    $this->assertSame('<p>#cache enabled, GET</p><p>This is a rendered placeholder!</p>', (string) $element['#markup'], '#markup is overridden.');
     $expected_js_settings = [
       'foo' => 'bar',
       'dynamic_animal' => $args[0],
@@ -321,9 +321,9 @@ public function testCacheableParent($test_element, $args, $placeholder_cid_keys,
     $element['#cache'] = ['keys' => ['placeholder_test_GET']];
     $element['#prefix'] = '<p>#cache enabled, GET</p>';
     $output = $this->renderer->renderRoot($element);
-    $this->assertSame('<p>#cache enabled, GET</p><p>This is a rendered placeholder!</p>', $output, 'Output is overridden.');
+    $this->assertSame('<p>#cache enabled, GET</p><p>This is a rendered placeholder!</p>', (string) $output, 'Output is overridden.');
     $this->assertFalse(isset($element['#printed']), 'Cache hit');
-    $this->assertSame('<p>#cache enabled, GET</p><p>This is a rendered placeholder!</p>', $element['#markup'], '#markup is overridden.');
+    $this->assertSame('<p>#cache enabled, GET</p><p>This is a rendered placeholder!</p>', (string) $element['#markup'], '#markup is overridden.');
     $expected_js_settings = [
       'foo' => 'bar',
       'dynamic_animal' => $args[0],
@@ -351,9 +351,9 @@ public function testCacheableParentWithPostRequest($test_element, $args) {
     $element['#cache'] = ['keys' => ['placeholder_test_POST']];
     $element['#prefix'] = '<p>#cache enabled, POST</p>';
     $output = $this->renderer->renderRoot($element);
-    $this->assertSame('<p>#cache enabled, POST</p><p>This is a rendered placeholder!</p>', $output, 'Output is overridden.');
+    $this->assertSame('<p>#cache enabled, POST</p><p>This is a rendered placeholder!</p>', (string) $output, 'Output is overridden.');
     $this->assertTrue(isset($element['#printed']), 'No cache hit');
-    $this->assertSame('<p>#cache enabled, POST</p><p>This is a rendered placeholder!</p>', $element['#markup'], '#markup is overridden.');
+    $this->assertSame('<p>#cache enabled, POST</p><p>This is a rendered placeholder!</p>', (string) $element['#markup'], '#markup is overridden.');
     $expected_js_settings = [
       'foo' => 'bar',
       'dynamic_animal' => $args[0],
@@ -383,7 +383,7 @@ public function testRecursivePlaceholder() {
 
     $output = $this->renderer->renderRoot($element);
     $this->assertEquals('<p>This is a rendered placeholder!</p>', $output, 'The output has been modified by the indirect, recursive placeholder #lazy_builder callback.');
-    $this->assertSame($element['#markup'], '<p>This is a rendered placeholder!</p>', '#markup is overridden by the indirect, recursive placeholder #lazy_builder callback.');
+    $this->assertSame((string) $element['#markup'], '<p>This is a rendered placeholder!</p>', '#markup is overridden by the indirect, recursive placeholder #lazy_builder callback.');
     $expected_js_settings = [
       'dynamic_animal' => $args[0],
     ];
@@ -546,9 +546,9 @@ public function testRenderChildrenPlaceholdersDifferentArguments() {
 </details></div>
 </details>
 HTML;
-    $this->assertSame($expected_output, $output, 'Output is not overridden.');
+    $this->assertSame($expected_output, (string) $output, 'Output is not overridden.');
     $this->assertTrue(isset($element['#printed']), 'No cache hit');
-    $this->assertSame($expected_output, $output, '#markup is not overridden.');
+    $this->assertSame($expected_output, (string) $element['#markup'], '#markup is not overridden.');
     $expected_js_settings = [
       'foo' => 'bar',
       'dynamic_animal' => [$args_1[0] => TRUE, $args_2[0] => TRUE, $args_3[0] => TRUE],
@@ -595,7 +595,7 @@ public function testRenderChildrenPlaceholdersDifferentArguments() {
     // GET request: #cache enabled, cache hit.
     $element = $test_element;
     $output = $this->renderer->renderRoot($element);
-    $this->assertSame($expected_output, $output, 'Output is not overridden.');
+    $this->assertSame($expected_output, (string) $output, 'Output is not overridden.');
     $this->assertFalse(isset($element['#printed']), 'Cache hit');
     $this->assertSame($element['#attached']['drupalSettings'], $expected_js_settings, '#attached is modified; both the original JavaScript setting and the ones added by each placeholder #lazy_builder callback exist.');
 
@@ -604,8 +604,8 @@ public function testRenderChildrenPlaceholdersDifferentArguments() {
     unset($test_element['#cache']);
     $element = $test_element;
     $output = $this->renderer->renderRoot($element);
-    $this->assertSame($expected_output, $output, 'Output is not overridden.');
-    $this->assertSame($expected_output, $output, '#markup is not overridden.');
+    $this->assertSame($expected_output, (string) $output, 'Output is not overridden.');
+    $this->assertSame($expected_output, (string) $element['#markup'], '#markup is not overridden.');
     $this->assertSame($element['#attached']['drupalSettings'], $expected_js_settings, '#attached is modified; both the original JavaScript setting and the ones added by each #lazy_builder callback exist.');
   }
 
diff --git a/core/tests/Drupal/Tests/Core/Render/RendererTest.php b/core/tests/Drupal/Tests/Core/Render/RendererTest.php
index a2f91ea..8439c0a 100644
--- a/core/tests/Drupal/Tests/Core/Render/RendererTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/RendererTest.php
@@ -10,6 +10,7 @@
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Cache\CacheableDependencyInterface;
 use Drupal\Core\Render\Element;
+use Drupal\Core\Render\SafeString;
 use Drupal\Core\Template\Attribute;
 
 /**
@@ -43,7 +44,7 @@ public function testRenderBasic($build, $expected, callable $setup_code = NULL)
       $setup_code();
     }
 
-    $this->assertSame($expected, $this->renderer->render($build));
+    $this->assertSame($expected, (string) $this->renderer->render($build));
   }
 
   /**
@@ -470,10 +471,10 @@ protected function assertAccess($build, $access) {
     $sensitive_content = $this->randomContextValue();
     $build['#markup'] = $sensitive_content;
     if ($access) {
-      $this->assertSame($sensitive_content, $this->renderer->render($build));
+      $this->assertSame($sensitive_content, (string) $this->renderer->render($build));
     }
     else {
-      $this->assertSame('', $this->renderer->render($build));
+      $this->assertSame('', (string) $this->renderer->render($build));
     }
   }
 
@@ -653,8 +654,8 @@ public function testRenderCacheProperties(array $expected_results) {
       ],
       // Collect expected property names.
       '#cache_properties' => array_keys(array_filter($expected_results)),
-      'child1' => ['#markup' => 1],
-      'child2' => ['#markup' => 2],
+      'child1' => ['#markup' => new SafeString(1, 'UTF-8')],
+      'child2' => ['#markup' => new SafeString(2, 'UTF-8')],
       '#custom_property' => ['custom_value'],
     ];
     $this->renderer->render($element);
diff --git a/core/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index 9f8bc4a..3c2ce61 100644
--- a/core/themes/engines/twig/twig.engine
+++ b/core/themes/engines/twig/twig.engine
@@ -7,6 +7,7 @@
 
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Core\Extension\Extension;
+use Drupal\Core\Render\SafeString;
 
 /**
  * Implements hook_theme().
@@ -103,7 +104,8 @@ function twig_render_template($template_file, array $variables) {
     $output['debug_info']   .= "\n<!-- BEGIN OUTPUT from '" . SafeMarkup::checkPlain($template_file) . "' -->\n";
     $output['debug_suffix'] .= "\n<!-- END OUTPUT from '" . SafeMarkup::checkPlain($template_file) . "' -->\n\n";
   }
-  return SafeMarkup::set(implode('', $output));
+  // This output has already been rendered and is therefore considered safe.
+  return new SafeString(implode('', $output), 'UTF-8');
 }
 
 /**