diff --git a/core/includes/common.inc b/core/includes/common.inc
index f30bcdf..75977c4 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -28,6 +28,7 @@
 use Drupal\Core\EventSubscriber\HtmlViewSubscriber;
 use Drupal\Core\Routing\GeneratorNotInitializedException;
 use Drupal\Core\Template\Attribute;
+use Drupal\Core\Template\SafeMarkup;
 use Drupal\Core\Render\Element;
 use Drupal\Core\Session\AnonymousUserSession;
 
@@ -915,7 +916,11 @@ function l($text, $path, array $options = array()) {
   // Sanitize the link text if necessary.
   $text = $variables['options']['html'] ? $variables['text'] : String::checkPlain($variables['text']);
 
-  return '<a href="' . $url . '"' . $attributes . '>' . $text . '</a>';
+  // $attributes and $url are known to be safe. Inline the safe check instead
+  // of using String::Concat() for performance reasons.
+  $safe = $text instanceof SafeMarkup;
+  $link = '<a href="' . $url . '"' . $attributes . '>' . $text . '</a>';
+  return $safe ? new SafeMarkup($link) : $link;
 }
 
 /**
@@ -1468,6 +1473,9 @@ function drupal_html_class($class) {
   // static instead of drupal_static().
   static $classes = array();
 
+  // @todo Needs safe makrup preservation or should be cast to string before
+  // arg is passed to this function?
+  $class = (string) $class;
   if (!isset($classes[$class])) {
     $classes[$class] = drupal_clean_css_identifier(drupal_strtolower($class));
   }
@@ -2857,6 +2865,7 @@ function drupal_pre_render_conditional_comments($elements) {
  *     - meta: To provide meta information, such as a page refresh.
  *     - link: To refer to stylesheets and other contextual information.
  *     - script: To load JavaScript.
+ *     This is not HTML escaped, do not pass in user input.
  *   - #attributes: (optional) An array of HTML attributes to apply to the
  *     tag.
  *   - #value: (optional) A string containing tag content, such as inline
@@ -2869,7 +2878,9 @@ function drupal_pre_render_conditional_comments($elements) {
 function drupal_pre_render_html_tag($element) {
   $attributes = isset($element['#attributes']) ? new Attribute($element['#attributes']) : '';
   if (!isset($element['#value'])) {
-    $markup = '<' . $element['#tag'] . $attributes . " />\n";
+    // Attributes are safe and we are assuming people don't use this function
+    // and second they don't pass unsafe variables to #tag.
+    $markup = new SafeMarkup('<' . $element['#tag'] . $attributes . " />\n");
   }
   else {
     $markup = '<' . $element['#tag'] . $attributes . '>';
@@ -2881,6 +2892,8 @@ function drupal_pre_render_html_tag($element) {
       $markup .= $element['#value_suffix'];
     }
     $markup .= '</' . $element['#tag'] . ">\n";
+    // @TODO Creating safe markup, avoid if possible!
+    $markup = new SafeMarkup($markup);
   }
   if (!empty($element['#noscript'])) {
     $element['#markup'] = '<noscript>' . $markup . '</noscript>';
@@ -3380,12 +3393,29 @@ function drupal_render(&$elements, $is_recursive_call = FALSE) {
     $theme_is_implemented = ($elements['#children'] !== FALSE);
   }
 
+  // Assume still safe unless the element or it's __toString return value aren't
+  // instances of Twig_Markup.
+  $safe = TRUE;
+  $get_string = function ($element) use (&$safe) {
+    if (is_object($element) && method_exists($element, '__toString')) {
+      $markup = $element->__toString();
+    }
+    else {
+      $markup = $element;
+    }
+    $safe = $safe && (!$markup || $element instanceof \Twig_Markup || $markup instanceof \Twig_Markup);
+    return $markup;
+  };
+
   // If #theme is not implemented or #render_children is set and the element has
   // an empty #children attribute, render the children now. This is the same
   // process as drupal_render_children() but is inlined for speed.
   if ((!$theme_is_implemented || isset($elements['#render_children'])) && empty($elements['#children'])) {
     foreach ($children as $key) {
-      $elements['#children'] .= drupal_render($elements[$key], TRUE);
+      $elements['#children'] .= $get_string(drupal_render($elements[$key], TRUE));
+    }
+    if ($safe) {
+      $elements['#children'] = new SafeMarkup($elements['#children']);
     }
   }
 
@@ -3397,7 +3427,10 @@ function drupal_render(&$elements, $is_recursive_call = FALSE) {
   // required. Eventually #theme_wrappers will expect both #markup and
   // #children to be a single string as #children.
   if (!$theme_is_implemented && isset($elements['#markup'])) {
-    $elements['#children'] = $elements['#markup'] . $elements['#children'];
+    $elements['#children'] = $get_string($elements['#markup']) . $get_string($elements['#children']);
+    if ($safe) {
+      $elements['#children'] = new SafeMarkup($elements['#children']);
+    }
   }
 
   // Let the theme functions in #theme_wrappers add markup around the rendered
@@ -3481,7 +3514,7 @@ function drupal_render(&$elements, $is_recursive_call = FALSE) {
   }
 
   $elements['#printed'] = TRUE;
-  return $elements['#markup'];
+  return $safe ? new SafeMarkup($elements['#markup']) : $elements['#markup'];
 }
 
 /**
@@ -3503,13 +3536,13 @@ function drupal_render_children(&$element, $children_keys = NULL) {
   if ($children_keys === NULL) {
     $children_keys = Element::children($element);
   }
-  $output = '';
+  $markups = array();
   foreach ($children_keys as $key) {
     if (!empty($element[$key])) {
-      $output .= drupal_render($element[$key]);
+      $markups[] = drupal_render($element[$key]);
     }
   }
-  return $output;
+  return String::implode('', $markups);
 }
 
 /**
diff --git a/core/includes/form.inc b/core/includes/form.inc
index 42206a4..8bc313c 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -15,6 +15,7 @@
 use Drupal\Core\Language\Language;
 use Drupal\Core\Render\Element;
 use Drupal\Core\Template\Attribute;
+use Drupal\Core\Template\SafeMarkup;
 use Drupal\Core\Utility\Color;
 use Symfony\Component\HttpFoundation\RedirectResponse;
 
@@ -983,7 +984,7 @@ function form_select_options($element, $choices = NULL) {
       $options .= form_select_options($element, $choice);
       $options .= '</optgroup>';
     }
-    elseif (is_object($choice)) {
+    elseif (is_object($choice) && isset($choice->option)) {
       $options .= form_select_options($element, $choice->option);
     }
     else {
@@ -998,7 +999,7 @@ function form_select_options($element, $choices = NULL) {
       $options .= '<option value="' . String::checkPlain($key) . '"' . $selected . '>' . String::checkPlain($choice) . '</option>';
     }
   }
-  return $options;
+  return new SafeMarkup($options);
 }
 
 /**
@@ -1659,7 +1660,7 @@ function theme_tableselect($variables) {
       foreach ($element['#header'] as $fieldname => $title) {
         // A row cell can span over multiple headers, which means less row cells
         // than headers could be present.
-        if (isset($element['#options'][$key][$fieldname])) {
+        if (isset($element['#options'][$key][$fieldname]) && is_array($element['#options'][$key][$fieldname])) {
           // A header can span over multiple cells and in this case the cells
           // are passed in an array. The order of this array determines the
           // order in which they are added.
@@ -2348,7 +2349,10 @@ function template_preprocess_input(&$variables) {
 function theme_input($variables) {
   $element = $variables['element'];
   $attributes = $variables['attributes'];
-  return '<input' . $attributes . ' />' . drupal_render_children($element);
+  $children = drupal_render_children($element);
+  $safe = $children instanceof SafeMarkup;
+  $return = '<input' . $attributes . ' />' . $children;
+  return $safe ? new SafeMarkup($return) : $return;
 }
 
 /**
@@ -2976,8 +2980,6 @@ function theme_form_element_label($variables) {
     return '';
   }
 
-  $title = Xss::filterAdmin($element['#title']);
-
   $attributes = array();
   // Style the label as class option to display inline with the element.
   if ($element['#title_display'] == 'after') {
@@ -3008,7 +3010,10 @@ function theme_form_element_label($variables) {
     $attributes['class'][] = 'form-required';
   }
 
-  return '<label' . new Attribute($attributes) . '>' . $title . '</label>';
+  $title = Xss::filterAdmin($element['#title']);
+
+  // Attribute is always safe and title is too so mark this safe.
+  return new SafeMarkup('<label' . new Attribute($attributes) . '>' . $title . '</label>');
 }
 
 /**
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index e44f13d..b4ebc20 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -21,6 +21,7 @@
 use Drupal\Core\Page\MetaElement;
 use Drupal\Core\Template\Attribute;
 use Drupal\Core\Template\RenderWrapper;
+use Drupal\Core\Template\SafeMarkup;
 use Drupal\Core\Theme\ThemeSettings;
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Render\Element;
@@ -650,7 +651,7 @@ function _theme($hook, $variables = array()) {
 
   // restore path_to_theme()
   $theme_path = $temp;
-  return (string) $output;
+  return $output;
 }
 
 /**
@@ -2052,7 +2053,10 @@ function template_preprocess_html(&$variables) {
 
   // Wrap function calls in an object so they can be called when printed.
   $variables['head'] = new RenderWrapper(function() use ($page) {
-    return implode("\n", $page->getMetaElements()) . implode("\n", $page->getLinkElements());
+    // HeadElement itself is safe and MetaElement and LinkElement both extend
+    // it so each element are safe. The result of concatenating a lot of them
+    // together is also safe.
+    return new SafeMarkup(implode("\n", $page->getMetaElements()) . implode("\n", $page->getLinkElements()));
   });
   $variables['styles'] = new RenderWrapper('drupal_get_css');
   $variables['scripts'] = new RenderWrapper('drupal_get_js');
diff --git a/core/lib/Drupal/Component/Utility/String.php b/core/lib/Drupal/Component/Utility/String.php
index 9796a5f..3cd6784 100644
--- a/core/lib/Drupal/Component/Utility/String.php
+++ b/core/lib/Drupal/Component/Utility/String.php
@@ -7,6 +7,8 @@
 
 namespace Drupal\Component\Utility;
 
+use Drupal\Core\Template\SafeMarkup;
+
 /**
  * Provides helpers to operate on strings.
  *
@@ -31,7 +33,7 @@ class String {
    * @see drupal_validate_utf8()
    */
   public static function checkPlain($text) {
-    return htmlspecialchars($text, ENT_QUOTES, 'UTF-8');
+    return $text instanceof SafeMarkup ? $text : new SafeMarkup(htmlspecialchars($text, ENT_QUOTES, 'UTF-8'));
   }
 
   /**
@@ -65,7 +67,8 @@ public static function decodeEntities($text) {
    * addition to formatting it.
    *
    * @param $string
-   *   A string containing placeholders.
+   *   A string containing placeholders. The string itself is not escaped, any
+   *   unsafe content must be in $args and inserted via placeholders.
    * @param $args
    *   An associative array of replacements to make. Occurrences in $string of
    *   any key in $args are replaced with the corresponding value, after
@@ -108,7 +111,7 @@ public static function format($string, array $args = array()) {
           // Pass-through.
       }
     }
-    return strtr($string, $args);
+    return new SafeMarkup(strtr($string, $args));
   }
 
   /**
@@ -123,7 +126,34 @@ public static function format($string, array $args = array()) {
    *   The formatted text (html).
    */
   public static function placeholder($text) {
-    return '<em class="placeholder">' . static::checkPlain($text) . '</em>';
+    return new SafeMarkup('<em class="placeholder">' . static::checkPlain($text) . '</em>');
   }
 
+  /**
+   * @todo.
+   */
+  public static function concat() {
+    return static::implode('', func_get_args());
+  }
+
+  /**
+   * @todo.
+   */
+  public static function implode($delimiter, array $array) {
+    $real_delimiter = '';
+    $safe = TRUE;
+    $imploded = '';
+    foreach ($array as $string) {
+      $safe = $safe && (!$string || $string instanceof SafeMarkup);
+      $imploded .= $real_delimiter . $string;
+      $real_delimiter = $delimiter;
+    }
+    return $safe ? new SafeMarkup($imploded) : $imploded;
+  }
+
+  public static function strReplace($search, $replace, $subject, &$count = NULL) {
+    $safe = $subject instanceof SafeMarkup;
+    $replacement = str_replace($search, $replace, $subject, $count);
+    return $safe ? new SafeMarkup($replacement) : $replacement;
+  }
 }
diff --git a/core/lib/Drupal/Component/Utility/Xss.php b/core/lib/Drupal/Component/Utility/Xss.php
index dc49913..dd0df76 100644
--- a/core/lib/Drupal/Component/Utility/Xss.php
+++ b/core/lib/Drupal/Component/Utility/Xss.php
@@ -7,6 +7,8 @@
 
 namespace Drupal\Component\Utility;
 
+use Drupal\Core\Template\SafeMarkup;
+
 /**
  * Provides helper to filter for cross-site scripting.
  *
@@ -90,7 +92,7 @@ public static function filter($string, $html_tags = array('a', 'em', 'strong', '
     $splitter = function ($matches) use ($html_tags, $mode) {
       return static::split($matches[1], $html_tags, $mode);
     };
-    return preg_replace_callback('%
+    return new SafeMarkup(preg_replace_callback('%
       (
       <(?=[^a-zA-Z!/])  # a lone <
       |                 # or
@@ -99,7 +101,7 @@ public static function filter($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/Config/StorableConfigBase.php b/core/lib/Drupal/Core/Config/StorableConfigBase.php
index 18c0f07..c45092b 100644
--- a/core/lib/Drupal/Core/Config/StorableConfigBase.php
+++ b/core/lib/Drupal/Core/Config/StorableConfigBase.php
@@ -9,6 +9,7 @@
 
 use Drupal\Component\Utility\String;
 use Drupal\Core\Config\Schema\SchemaIncompleteException;
+use Drupal\Core\Template\SafeMarkup;
 use Drupal\Core\TypedData\PrimitiveInterface;
 use Drupal\Core\TypedData\Type\FloatInterface;
 use Drupal\Core\TypedData\Type\IntegerInterface;
@@ -167,6 +168,9 @@ protected function validateValue($key, $value) {
    *   Exception on unsupported/undefined data type deducted.
    */
   protected function castValue($key, $value) {
+    if ($value instanceof SafeMarkup) {
+      $value = $value->__toString();
+    }
     if ($value === NULL) {
       $value = NULL;
     }
diff --git a/core/lib/Drupal/Core/CoreServiceProvider.php b/core/lib/Drupal/Core/CoreServiceProvider.php
index fe23fb6..99784f7 100644
--- a/core/lib/Drupal/Core/CoreServiceProvider.php
+++ b/core/lib/Drupal/Core/CoreServiceProvider.php
@@ -101,7 +101,7 @@ public static function registerTwig(ContainerBuilder $container) {
         'cache' => drupal_installation_attempted() ? FALSE : Settings::get('twig_cache', TRUE),
         // @todo Remove in followup issue
         // @see http://drupal.org/node/1712444.
-        'autoescape' => FALSE,
+        'autoescape' => TRUE,
         'debug' => Settings::get('twig_debug', FALSE),
         'auto_reload' => Settings::get('twig_auto_reload', NULL),
       ))
diff --git a/core/lib/Drupal/Core/Database/Install/Tasks.php b/core/lib/Drupal/Core/Database/Install/Tasks.php
index c4d88d2..6d0dfe7 100644
--- a/core/lib/Drupal/Core/Database/Install/Tasks.php
+++ b/core/lib/Drupal/Core/Database/Install/Tasks.php
@@ -92,14 +92,14 @@ protected function hasPdoDriver() {
    * Assert test as failed.
    */
   protected function fail($message) {
-    $this->results[$message] = FALSE;
+    $this->results[(string) $message] = FALSE;
   }
 
   /**
    * Assert test as a pass.
    */
   protected function pass($message) {
-    $this->results[$message] = TRUE;
+    $this->results[(string) $message] = TRUE;
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Extension/ThemeHandler.php b/core/lib/Drupal/Core/Extension/ThemeHandler.php
index dbe2c78..8354bea 100644
--- a/core/lib/Drupal/Core/Extension/ThemeHandler.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandler.php
@@ -626,6 +626,8 @@ public function getName($theme) {
     if (!isset($themes[$theme])) {
       throw new \InvalidArgumentException(String::format('Requested the name of a non-existing theme @theme', array('@theme' => $theme)));
     }
+    // @todo For testing please remove before commit.
+    return \Drupal::config('system.site')->get('name');
     return String::checkPlain($themes[$theme]->info['name']);
   }
 
diff --git a/core/lib/Drupal/Core/Form/OptGroup.php b/core/lib/Drupal/Core/Form/OptGroup.php
index f94d62d..21e9855 100644
--- a/core/lib/Drupal/Core/Form/OptGroup.php
+++ b/core/lib/Drupal/Core/Form/OptGroup.php
@@ -43,7 +43,7 @@ public static function flattenOptions(array $array) {
    */
   protected static function doFlattenOptions(array $array, array &$options) {
     foreach ($array as $key => $value) {
-      if (is_object($value)) {
+      if (is_object($value) && isset($value->option)) {
         static::doFlattenOptions($value->option, $options);
       }
       elseif (is_array($value)) {
diff --git a/core/lib/Drupal/Core/Page/HeadElement.php b/core/lib/Drupal/Core/Page/HeadElement.php
index 85055ad..70cf2b7 100644
--- a/core/lib/Drupal/Core/Page/HeadElement.php
+++ b/core/lib/Drupal/Core/Page/HeadElement.php
@@ -8,11 +8,12 @@
 namespace Drupal\Core\Page;
 
 use Drupal\Core\Template\Attribute;
+use Drupal\Core\Template\SafeMarkup;
 
 /**
  * This class represents an HTML element that appears in the HEAD tag.
  */
-class HeadElement {
+class HeadElement extends SafeMarkup {
 
   /**
    * An array of attributes for this element.
diff --git a/core/lib/Drupal/Core/StringTranslation/TranslationManager.php b/core/lib/Drupal/Core/StringTranslation/TranslationManager.php
index bd1b3b3..68731c7 100644
--- a/core/lib/Drupal/Core/StringTranslation/TranslationManager.php
+++ b/core/lib/Drupal/Core/StringTranslation/TranslationManager.php
@@ -10,6 +10,7 @@
 use Drupal\Component\Utility\String;
 use Drupal\Core\Language\LanguageManagerInterface;
 use Drupal\Core\StringTranslation\Translator\TranslatorInterface;
+use Drupal\Core\Template\SafeMarkup;
 
 /**
  * Defines a chained translation implementation combining multiple translators.
@@ -140,7 +141,7 @@ public function translate($string, array $args = array(), array $options = array
     $string = $translation === FALSE ? $string : $translation;
 
     if (empty($args)) {
-      return $string;
+      return new SafeMarkup($string);
     }
     else {
       return String::format($string, $args);
diff --git a/core/lib/Drupal/Core/Template/Attribute.php b/core/lib/Drupal/Core/Template/Attribute.php
index ead5d05..7432b6d 100644
--- a/core/lib/Drupal/Core/Template/Attribute.php
+++ b/core/lib/Drupal/Core/Template/Attribute.php
@@ -31,7 +31,7 @@
  *  // Produces <cat class="cat black-cat white-cat black-white-cat" id="socks">
  * @endcode
  */
-class Attribute implements \ArrayAccess, \IteratorAggregate {
+class Attribute extends \Twig_Markup implements \ArrayAccess, \IteratorAggregate {
 
   /**
    * Stores the attribute data.
@@ -86,9 +86,12 @@ protected function createAttributeValue($name, $value) {
     elseif (is_bool($value)) {
       $value = new AttributeBoolean($name, $value);
     }
-    elseif (!is_object($value)) {
+    elseif (!is_object($value) || $value instanceof SafeMarkup) {
       $value = new AttributeString($name, $value);
     }
+    elseif (!method_exists($value, 'render')) {
+      throw new \Exception('boo!');
+    }
     return $value;
   }
 
diff --git a/core/lib/Drupal/Core/Template/AttributeArray.php b/core/lib/Drupal/Core/Template/AttributeArray.php
index 95e0ef3..4d8145f 100644
--- a/core/lib/Drupal/Core/Template/AttributeArray.php
+++ b/core/lib/Drupal/Core/Template/AttributeArray.php
@@ -66,7 +66,7 @@ public function offsetExists($offset) {
    * Implements the magic __toString() method.
    */
   public function __toString() {
-    return String::checkPlain(implode(' ', $this->value));
+    return (string) String::checkPlain(implode(' ', $this->value));
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Template/AttributeBoolean.php b/core/lib/Drupal/Core/Template/AttributeBoolean.php
index 4e9ea67..b5a0957 100644
--- a/core/lib/Drupal/Core/Template/AttributeBoolean.php
+++ b/core/lib/Drupal/Core/Template/AttributeBoolean.php
@@ -42,7 +42,7 @@ public function render() {
    * Implements the magic __toString() method.
    */
   public function __toString() {
-    return $this->value === FALSE ? '' : String::checkPlain($this->name);
+    return $this->value === FALSE ? '' : (string) String::checkPlain($this->name);
   }
 
 }
diff --git a/core/lib/Drupal/Core/Template/AttributeString.php b/core/lib/Drupal/Core/Template/AttributeString.php
index 07211be..e1c69dc 100644
--- a/core/lib/Drupal/Core/Template/AttributeString.php
+++ b/core/lib/Drupal/Core/Template/AttributeString.php
@@ -30,7 +30,7 @@ class AttributeString extends AttributeValueBase {
    * Implements the magic __toString() method.
    */
   public function __toString() {
-    return String::checkPlain($this->value);
+    return (string) String::checkPlain($this->value);
   }
 
 }
diff --git a/core/lib/Drupal/Core/Utility/LinkGenerator.php b/core/lib/Drupal/Core/Utility/LinkGenerator.php
index b547d95..3e64c1a 100644
--- a/core/lib/Drupal/Core/Utility/LinkGenerator.php
+++ b/core/lib/Drupal/Core/Utility/LinkGenerator.php
@@ -11,8 +11,9 @@
 use Drupal\Component\Utility\String;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Path\AliasManagerInterface;
-use Drupal\Core\Template\Attribute;
 use Drupal\Core\Routing\UrlGeneratorInterface;
+use Drupal\Core\Template\Attribute;
+use Drupal\Core\Template\SafeMarkup;
 use Drupal\Core\Url;
 
 /**
@@ -123,7 +124,11 @@ public function generateFromUrl($text, Url $url) {
     // Sanitize the link text if necessary.
     $text = $variables['options']['html'] ? $variables['text'] : String::checkPlain($variables['text']);
 
-    return '<a href="' . $url . '"' . $attributes . '>' . $text . '</a>';
+    // $attributes and $url are known to be safe. Inline the safe check instead
+    // of using String::Concat() for performance reasons.
+    $safe = $text instanceof SafeMarkup;
+    $link = '<a href="' . $url . '"' . $attributes . '>' . $text . '</a>';
+    return $safe ? new SafeMarkup($link) : $link;
   }
 
   /**
diff --git a/core/modules/contextual/lib/Drupal/contextual/ContextualController.php b/core/modules/contextual/lib/Drupal/contextual/ContextualController.php
index aeee4a6..fcc47de 100644
--- a/core/modules/contextual/lib/Drupal/contextual/ContextualController.php
+++ b/core/modules/contextual/lib/Drupal/contextual/ContextualController.php
@@ -44,7 +44,10 @@ public function render(Request $request) {
         '#type' => 'contextual_links',
         '#contextual_links' => _contextual_id_to_links($id),
       );
-      $rendered[$id] = drupal_render($element);
+      // Cast any returned safe markup to string so that it is created
+      // as correct JSON.
+      // @todo Maybe check safeness and checkplain the results?
+      $rendered[$id] = (string) drupal_render($element);
     }
 
     return new JsonResponse($rendered);
diff --git a/core/modules/field/lib/Drupal/field/Plugin/views/field/Field.php b/core/modules/field/lib/Drupal/field/Plugin/views/field/Field.php
index 84b587a..0963e34 100644
--- a/core/modules/field/lib/Drupal/field/Plugin/views/field/Field.php
+++ b/core/modules/field/lib/Drupal/field/Plugin/views/field/Field.php
@@ -7,12 +7,14 @@
 
 namespace Drupal\field\Plugin\views\field;
 
+use Drupal\Component\Utility\String;
 use Drupal\Component\Utility\Xss;
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Entity\EntityManagerInterface;
 use Drupal\Core\Field\FieldDefinition;
 use Drupal\Core\Entity\EntityStorageInterface;
 use Drupal\Core\Render\Element;
+use Drupal\Core\Template\SafeMarkup;
 use Drupal\Core\Entity\ContentEntityDatabaseStorage;
 use Drupal\Core\Field\FieldDefinitionInterface;
 use Drupal\Core\Field\FormatterPluginManager;
@@ -686,11 +688,11 @@ public function submitGroupByForm(&$form, &$form_state) {
   protected function renderItems($items) {
     if (!empty($items)) {
       if (!$this->options['group_rows']) {
-        return implode('', $items);
+        return String::implode('', $items);
       }
 
       if ($this->options['multi_type'] == 'separator') {
-        return implode(Xss::filterAdmin($this->options['separator']), $items);
+        return String::implode(Xss::filterAdmin($this->options['separator']), $items);
       }
       else {
         $item_list = array(
diff --git a/core/modules/field_ui/lib/Drupal/field_ui/FieldConfigListBuilder.php b/core/modules/field_ui/lib/Drupal/field_ui/FieldConfigListBuilder.php
index e1d96c6..4fc313d 100644
--- a/core/modules/field_ui/lib/Drupal/field_ui/FieldConfigListBuilder.php
+++ b/core/modules/field_ui/lib/Drupal/field_ui/FieldConfigListBuilder.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\field_ui;
 
+use Drupal\Component\Utility\String;
 use Drupal\Core\Config\Entity\ConfigEntityListBuilder;
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Entity\EntityManagerInterface;
@@ -117,7 +118,7 @@ public function buildRow(EntityInterface $field) {
         $usage[] = $this->bundles[$field->entity_type][$bundle]['label'];
       }
     }
-    $row['data']['usage'] = implode(', ', $usage);
+    $row['data']['usage'] = String::implode(', ', $usage);
     return $row;
   }
 
diff --git a/core/modules/filter/filter.module b/core/modules/filter/filter.module
index 3efe9ad..cd17f6a 100644
--- a/core/modules/filter/filter.module
+++ b/core/modules/filter/filter.module
@@ -13,6 +13,7 @@
 use Drupal\Core\Render\Element;
 use Drupal\Core\Session\AccountInterface;
 use Drupal\Core\Template\Attribute;
+use Drupal\Core\Template\SafeMarkup;
 use Drupal\filter\FilterFormatInterface;
 use Drupal\filter\Plugin\FilterInterface;
 use Symfony\Component\HttpFoundation\Request;
@@ -419,7 +420,7 @@ function check_markup($text, $format_id = NULL, $langcode = '', $cache = FALSE,
     \Drupal::cache('filter')->set($cache_id, $text, Cache::PERMANENT, array('filter_format' => $format->id()));
   }
 
-  return $text;
+  return new SafeMarkup($text);
 }
 
 /**
diff --git a/core/modules/filter/templates/filter-guidelines.html.twig b/core/modules/filter/templates/filter-guidelines.html.twig
index 88a3b47..ecf9b94 100644
--- a/core/modules/filter/templates/filter-guidelines.html.twig
+++ b/core/modules/filter/templates/filter-guidelines.html.twig
@@ -20,6 +20,6 @@
  */
 #}
 <div{{ attributes }}>
-  <h4 class="label">{{ format.name|escape }}</h4>
+  <h4 class="label">{{ format.name }}</h4>
   {{ tips }}
 </div>
diff --git a/core/modules/node/node.install b/core/modules/node/node.install
index b075d93..7888e2b 100644
--- a/core/modules/node/node.install
+++ b/core/modules/node/node.install
@@ -7,6 +7,7 @@
 
 use Drupal\Component\Uuid\Uuid;
 use Drupal\Core\Language\Language;
+use Drupal\Core\Template\SafeMarkup;
 
 /**
  * Implements hook_requirements().
@@ -29,7 +30,9 @@ function node_requirements($phase) {
     $requirements['node_access'] = array(
       'title' => t('Node Access Permissions'),
       'value' => $value,
-      'description' => $description . ' ' . l(t('Rebuild permissions'), 'admin/reports/status/rebuild'),
+      // The results of t() is safe and so is the results of l(). Preserving
+      // safe object.
+      'description' => new SafeMarkup($description . ' ' . l(t('Rebuild permissions'), 'admin/reports/status/rebuild')),
     );
   }
   return $requirements;
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/Form/SimpletestResultsForm.php b/core/modules/simpletest/lib/Drupal/simpletest/Form/SimpletestResultsForm.php
index 299f305..2786a3a 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/Form/SimpletestResultsForm.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/Form/SimpletestResultsForm.php
@@ -9,6 +9,7 @@
 
 use Drupal\Core\Database\Connection;
 use Drupal\Core\Form\FormBase;
+use Drupal\Core\Template\SafeMarkup;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\HttpFoundation\RedirectResponse;
 
@@ -155,7 +156,8 @@ public function buildForm(array $form, array &$form_state, $test_id = NULL) {
       $rows = array();
       foreach ($assertions as $assertion) {
         $row = array();
-        $row[] = $assertion->message;
+        // @todo Need to preserve safe markup, not create it.
+        $row[] = new SafeMarkup($assertion->message);
         $row[] = $assertion->message_group;
         $row[] = drupal_basename($assertion->file);
         $row[] = $assertion->line;
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index 19dd044..1ebeb39 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -277,7 +277,7 @@ protected function assert($status, $message = '', $group = 'Other', array $calle
       'test_id' => $this->testId,
       'test_class' => get_class($this),
       'status' => $status,
-      'message' => $message,
+      'message' => (string) $message,
       'message_group' => $group,
       'function' => $caller['function'],
       'line' => $caller['line'],
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index 340b555..b97f8f5 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -23,6 +23,7 @@
 use Drupal\Core\StreamWrapper\PublicStream;
 use Drupal\Core\Datetime\DrupalDateTime;
 use Drupal\block\Entity\Block;
+use Drupal\Core\Template\SafeMarkup;
 use Symfony\Component\HttpFoundation\Request;
 
 /**
@@ -1628,6 +1629,9 @@ protected function drupalGetAJAX($path, array $options = array(), array $headers
    */
   protected function drupalPostForm($path, $edit, $submit, array $options = array(), array $headers = array(), $form_html_id = NULL, $extra_post = NULL) {
     $submit_matches = FALSE;
+    if ($submit instanceof SafeMarkup) {
+      $submit = (string) $submit;
+    }
     $ajax = is_array($submit);
     if (isset($path)) {
       $this->drupalGet($path, $options);
@@ -2759,7 +2763,7 @@ protected function assertRaw($raw, $message = '', $group = 'Other') {
     if (!$message) {
       $message = String::format('Raw "@raw" found', array('@raw' => $raw));
     }
-    return $this->assert(strpos($this->drupalGetContent(), $raw) !== FALSE, $message, $group);
+    return $this->assert(strpos($this->drupalGetContent(), (string) $raw) !== FALSE, $message, $group);
   }
 
   /**
@@ -2786,7 +2790,7 @@ protected function assertNoRaw($raw, $message = '', $group = 'Other') {
     if (!$message) {
       $message = String::format('Raw "@raw" not found', array('@raw' => $raw));
     }
-    return $this->assert(strpos($this->drupalGetContent(), $raw) === FALSE, $message, $group);
+    return $this->assert(strpos($this->drupalGetContent(), (string) $raw) === FALSE, $message, $group);
   }
 
   /**
@@ -2870,7 +2874,7 @@ protected function assertTextHelper($text, $message = '', $group, $not_exists) {
     if (!$message) {
       $message = !$not_exists ? String::format('"@text" found', array('@text' => $text)) : String::format('"@text" not found', array('@text' => $text));
     }
-    return $this->assert($not_exists == (strpos($this->plainTextContent, $text) === FALSE), $message, $group);
+    return $this->assert($not_exists == (strpos($this->plainTextContent, (string) $text) === FALSE), $message, $group);
   }
 
   /**
@@ -3684,6 +3688,7 @@ protected function assertMail($name, $value = '', $message = '', $group = 'E-mai
   protected function assertMailString($field_name, $string, $email_depth, $message = '', $group = 'Other') {
     $mails = $this->drupalGetMails();
     $string_found = FALSE;
+    $string = (string) $string;
     for ($i = count($mails) -1; $i >= count($mails) - $email_depth && $i >= 0; $i--) {
       $mail = $mails[$i];
       // Normalize whitespace, as we don't know what the mail system might have
diff --git a/core/modules/system/lib/Drupal/system/Form/ModulesUninstallForm.php b/core/modules/system/lib/Drupal/system/Form/ModulesUninstallForm.php
index 7438e48..23de0ba 100644
--- a/core/modules/system/lib/Drupal/system/Form/ModulesUninstallForm.php
+++ b/core/modules/system/lib/Drupal/system/Form/ModulesUninstallForm.php
@@ -10,6 +10,7 @@
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Form\FormBase;
 use Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface;
+use Drupal\Core\Template\SafeMarkup;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -114,7 +115,7 @@ public function buildForm(array $form, array &$form_state) {
     foreach ($uninstallable as $module) {
       $name = $module->info['name'] ?: $module->getName();
       $form['modules'][$module->getName()]['#module_name'] = $name;
-      $form['modules'][$module->getName()]['name']['#markup'] = $name;
+      $form['modules'][$module->getName()]['name']['#markup'] = new SafeMarkup($name);
       $form['modules'][$module->getName()]['description']['#markup'] = $this->t($module->info['description']);
 
       $form['uninstall'][$module->getName()] = array(
diff --git a/core/modules/system/lib/Drupal/system/Tests/Common/RenderTest.php b/core/modules/system/lib/Drupal/system/Tests/Common/RenderTest.php
index caa80a2..d3b21e3 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Common/RenderTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Common/RenderTest.php
@@ -277,7 +277,8 @@ function testDrupalRenderBasics() {
     );
 
     foreach($types as $type) {
-      $this->assertIdentical(drupal_render($type['value']), $type['expected'], '"' . $type['name'] . '" input rendered correctly by drupal_render().');
+      $value = drupal_render($type['value']);
+      $this->assertIdentical((string) $value, $type['expected'], '"' . $type['name'] . '" input rendered correctly by drupal_render().');
     }
   }
 
@@ -394,14 +395,14 @@ function testDrupalRenderThemeArguments() {
       '#theme' => 'common_test_foo',
     );
     // Test that defaults work.
-    $this->assertEqual(drupal_render($element), 'foobar', 'Defaults work');
+    $this->assertEqual((string) drupal_render($element), 'foobar', 'Defaults work');
     $element = array(
       '#theme' => 'common_test_foo',
       '#foo' => $this->randomName(),
       '#bar' => $this->randomName(),
     );
     // Tests that passing arguments to the theme function works.
-    $this->assertEqual(drupal_render($element), $element['#foo'] . $element['#bar'], 'Passing arguments to theme functions works');
+    $this->assertEqual((string) drupal_render($element), $element['#foo'] . $element['#bar'], 'Passing arguments to theme functions works');
   }
 
   /**
@@ -470,7 +471,7 @@ function testDrupalRenderPostRenderCache() {
     $element = $test_element;
     $element['#markup'] = '<p>#cache disabled</p>';
     $output = drupal_render($element);
-    $this->assertIdentical($output, '<p>overridden</p>', 'Output is overridden.');
+    $this->assertIdentical((string) $output, '<p>overridden</p>', 'Output is overridden.');
     $this->assertIdentical($element['#markup'], '<p>overridden</p>', '#markup is overridden.');
     $settings = $this->parseDrupalSettings(drupal_get_js());
     $this->assertIdentical($settings['foo'], 'bar', 'Original JavaScript setting is added to the page.');
@@ -486,7 +487,7 @@ function testDrupalRenderPostRenderCache() {
     $element['#cache'] = array('cid' => 'post_render_cache_test_GET');
     $element['#markup'] = '<p>#cache enabled, GET</p>';
     $output = drupal_render($element);
-    $this->assertIdentical($output, '<p>overridden</p>', 'Output is overridden.');
+    $this->assertIdentical((string) $output, '<p>overridden</p>', 'Output is overridden.');
     $this->assertTrue(isset($element['#printed']), 'No cache hit');
     $this->assertIdentical($element['#markup'], '<p>overridden</p>', '#markup is overridden.');
     $settings = $this->parseDrupalSettings(drupal_get_js());
@@ -509,7 +510,7 @@ function testDrupalRenderPostRenderCache() {
     $element['#cache'] = array('cid' => 'post_render_cache_test_GET');
     $element['#markup'] = '<p>#cache enabled, GET</p>';
     $output = drupal_render($element);
-    $this->assertIdentical($output, '<p>overridden</p>', 'Output is overridden.');
+    $this->assertIdentical((string) $output, '<p>overridden</p>', 'Output is overridden.');
     $this->assertFalse(isset($element['#printed']), 'Cache hit');
     $this->assertIdentical($element['#markup'], '<p>overridden</p>', '#markup is overridden.');
     $settings = $this->parseDrupalSettings(drupal_get_js());
@@ -526,7 +527,7 @@ function testDrupalRenderPostRenderCache() {
     $element['#cache'] = array('cid' => 'post_render_cache_test_POST');
     $element['#markup'] = '<p>#cache enabled, POST</p>';
     $output = drupal_render($element);
-    $this->assertIdentical($output, '<p>overridden</p>', 'Output is overridden.');
+    $this->assertIdentical((string) $output, '<p>overridden</p>', 'Output is overridden.');
     $this->assertTrue(isset($element['#printed']), 'No cache hit');
     $this->assertIdentical($element['#markup'], '<p>overridden</p>', '#markup is overridden.');
     $settings = $this->parseDrupalSettings(drupal_get_js());
@@ -589,7 +590,7 @@ function testDrupalRenderChildrenPostRenderCache() {
     );
     $element = $test_element;
     $output = drupal_render($element);
-    $this->assertIdentical($output, '<p>overridden</p>', 'Output is overridden.');
+    $this->assertIdentical((string) $output, '<p>overridden</p>', 'Output is overridden.');
     $this->assertTrue(isset($element['#printed']), 'No cache hit');
     $this->assertIdentical($element['#markup'], '<p>overridden</p>', '#markup is overridden.');
     $settings = $this->parseDrupalSettings(drupal_get_js());
@@ -635,7 +636,7 @@ function testDrupalRenderChildrenPostRenderCache() {
     drupal_static_reset('_drupal_add_js');
     $element = $test_element;
     $output = drupal_render($element);
-    $this->assertIdentical($output, '<p>overridden</p>', 'Output is overridden.');
+    $this->assertIdentical((string) $output, '<p>overridden</p>', 'Output is overridden.');
     $this->assertFalse(isset($element['#printed']), 'Cache hit');
     $settings = $this->parseDrupalSettings(drupal_get_js());
     $this->assertIdentical($settings['foo'], 'bar', 'Original JavaScript setting is added to the page.');
@@ -647,7 +648,7 @@ function testDrupalRenderChildrenPostRenderCache() {
     unset($test_element['#cache']);
     $element = $test_element;
     $output = drupal_render($element);
-    $this->assertIdentical($output, '<p>overridden</p>', 'Output is overridden.');
+    $this->assertIdentical((string) $output, '<p>overridden</p>', 'Output is overridden.');
     $this->assertIdentical($element['#markup'], '<p>overridden</p>', '#markup is overridden.');
     $settings = $this->parseDrupalSettings(drupal_get_js());
     $expected_settings = $context_1 + $context_2 + $context_3;
@@ -667,7 +668,7 @@ function testDrupalRenderChildrenPostRenderCache() {
     $element['#cache']['keys'] = array('simpletest', 'drupal_render', 'children_post_render_cache', 'nested_cache_parent');
     $element['child']['#cache']['keys'] = array('simpletest', 'drupal_render', 'children_post_render_cache', 'nested_cache_child');
     $output = drupal_render($element);
-    $this->assertIdentical($output, '<p>overridden</p>', 'Output is overridden.');
+    $this->assertIdentical((string) $output, '<p>overridden</p>', 'Output is overridden.');
     $this->assertTrue(isset($element['#printed']), 'No cache hit');
     $this->assertIdentical($element['#markup'], '<p>overridden</p>', '#markup is overridden.');
     $settings = $this->parseDrupalSettings(drupal_get_js());
@@ -742,7 +743,7 @@ function testDrupalRenderChildrenPostRenderCache() {
     $element = $test_element;
     $element['#cache']['keys'] = array('simpletest', 'drupal_render', 'children_post_render_cache', 'nested_cache_parent');
     $output = drupal_render($element);
-    $this->assertIdentical($output, '<p>overridden</p>', 'Output is overridden.');
+    $this->assertIdentical((string) $output, '<p>overridden</p>', 'Output is overridden.');
     $this->assertFalse(isset($element['#printed']), 'Cache hit');
     $settings = $this->parseDrupalSettings(drupal_get_js());
     $this->assertIdentical($settings['foo'], 'bar', 'Original JavaScript setting is added to the page.');
@@ -754,7 +755,7 @@ function testDrupalRenderChildrenPostRenderCache() {
     $element['child']['#cache']['keys'] = array('simpletest', 'drupal_render', 'children_post_render_cache', 'nested_cache_child');
     $element = $element['child'];
     $output = drupal_render($element);
-    $this->assertIdentical($output, '<p>overridden</p>', 'Output is overridden.');
+    $this->assertIdentical((string) $output, '<p>overridden</p>', 'Output is overridden.');
     $this->assertFalse(isset($element['#printed']), 'Cache hit');
     $settings = $this->parseDrupalSettings(drupal_get_js());
     $expected_settings = $context_2 + $context_3;
@@ -790,7 +791,7 @@ function testDrupalRenderRenderCachePlaceholder() {
     drupal_static_reset('_drupal_add_js');
     $element = $test_element;
     $output = drupal_render($element);
-    $this->assertIdentical($output, $expected_output, 'Placeholder was replaced in output');
+    $this->assertIdentical((string) $output, $expected_output, 'Placeholder was replaced in output');
     $settings = $this->parseDrupalSettings(drupal_get_js());
     $this->assertIdentical($settings['common_test'], $context, '#attached is modified; JavaScript setting is added to page.');
 
@@ -803,7 +804,7 @@ function testDrupalRenderRenderCachePlaceholder() {
     $element = $test_element;
     $element['#cache'] = array('cid' => 'render_cache_placeholder_test_GET');
     $output = drupal_render($element);
-    $this->assertIdentical($output, $expected_output, 'Placeholder was replaced in output');
+    $this->assertIdentical((string) $output, $expected_output, 'Placeholder was replaced in output');
     $this->assertTrue(isset($element['#printed']), 'No cache hit');
     $this->assertIdentical($element['#markup'], $expected_output, 'Placeholder was replaced in #markup.');
     $settings = $this->parseDrupalSettings(drupal_get_js());
@@ -840,7 +841,7 @@ function testDrupalRenderRenderCachePlaceholder() {
     $element = $test_element;
     $element['#cache'] = array('cid' => 'render_cache_placeholder_test_GET');
     $output = drupal_render($element);
-    $this->assertIdentical($output, $expected_output, 'Placeholder was replaced in output');
+    $this->assertIdentical((string) $output, $expected_output, 'Placeholder was replaced in output');
     $this->assertFalse(isset($element['#printed']), 'Cache hit');
     $this->assertIdentical($element['#markup'], $expected_output, 'Placeholder was replaced in #markup.');
     $settings = $this->parseDrupalSettings(drupal_get_js());
@@ -880,7 +881,7 @@ function testDrupalRenderChildElementRenderCachePlaceholder() {
     drupal_static_reset('_drupal_add_js');
     $element = $container;
     $output = drupal_render($element);
-    $this->assertIdentical($output, $expected_output, 'Placeholder was replaced in output');
+    $this->assertIdentical((string) $output, $expected_output, 'Placeholder was replaced in output');
     $settings = $this->parseDrupalSettings(drupal_get_js());
     $this->assertIdentical($settings['common_test'], $context, '#attached is modified; JavaScript setting is added to page.');
 
@@ -899,7 +900,7 @@ function testDrupalRenderChildElementRenderCachePlaceholder() {
     $element['#children'] = drupal_render($child, TRUE);
     // Eventually, drupal_render() gets called on the root element.
     $output = drupal_render($element);
-    $this->assertIdentical($output, $expected_output, 'Placeholder was replaced in output');
+    $this->assertIdentical((string) $output, $expected_output, 'Placeholder was replaced in output');
     $this->assertTrue(isset($element['#printed']), 'No cache hit');
     $this->assertIdentical($element['#markup'], $expected_output, 'Placeholder was replaced in #markup.');
     $settings = $this->parseDrupalSettings(drupal_get_js());
@@ -996,7 +997,7 @@ function testDrupalRenderChildElementRenderCachePlaceholder() {
     $child = &$element['test_element'];
     $element['#children'] = drupal_render($child, TRUE);
     $output = drupal_render($element);
-    $this->assertIdentical($output, $expected_output, 'Placeholder was replaced in output');
+    $this->assertIdentical((string) $output, $expected_output, 'Placeholder was replaced in output');
     $this->assertFalse(isset($element['#printed']), 'Cache hit');
     $this->assertIdentical($element['#markup'], $expected_output, 'Placeholder was replaced in #markup.');
     $settings = $this->parseDrupalSettings(drupal_get_js());
diff --git a/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
index 9ba1a1c..255832d 100644
--- a/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -5,11 +5,13 @@
  * Admin page callbacks for the system module.
  */
 
+use Drupal\Component\Utility\String;
 use Drupal\Component\Utility\Xss;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Extension\Extension;
 use Drupal\Core\Render\Element;
 use Drupal\Core\Template\Attribute;
+use Drupal\Core\Template\SafeMarkup;
 
 /**
  * Recursively check compatibility.
@@ -313,9 +315,12 @@ function theme_system_modules_uninstall($variables) {
     else {
       $disabled_message = '';
     }
+    $name = drupal_render($form['modules'][$module]['name']);
+    $safe = $name instanceof SafeMarkup;
+    $label = '<label for="' . $form['uninstall'][$module]['#id'] . '" class="module-name table-filter-text-source">' . $name . '</label>';
     $rows[] = array(
       array('data' => drupal_render($form['uninstall'][$module]), 'align' => 'center'),
-      '<label for="' . $form['uninstall'][$module]['#id'] . '" class="module-name table-filter-text-source">' . drupal_render($form['modules'][$module]['name']) . '</label>',
+      $safe ? new SafeMarkup($label) : $label,
       array('data' => drupal_render($form['modules'][$module]['description']) . $disabled_message, 'class' => array('description')),
     );
   }
@@ -326,11 +331,11 @@ function theme_system_modules_uninstall($variables) {
     '#rows' => $rows,
     '#empty' => t('No modules are available to uninstall.'),
   );
-  $output = drupal_render($form['filters']);
-  $output .= drupal_render($table);
-  $output .= drupal_render_children($form);
+  $filters = drupal_render($form['filters']);
+  $table = drupal_render($table);
+  $form = drupal_render_children($form);
 
-  return $output;
+  return String::concat($filters, $table, $form);
 }
 
 /**
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index a14cd7b..b8b4d6d 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -10,6 +10,7 @@
 use Drupal\Core\Language\Language;
 use Drupal\Core\Site\Settings;
 use Drupal\Core\StreamWrapper\PublicStream;
+use Drupal\Core\Template\SafeMarkup;
 
 /**
  * Implements hook_requirements().
@@ -56,7 +57,8 @@ function system_requirements($phase) {
   if (function_exists('phpinfo')) {
     $requirements['php'] = array(
       'title' => t('PHP'),
-      'value' => ($phase == 'runtime') ? $phpversion .' ('. l(t('more information'), 'admin/reports/status/php') .')' : $phpversion,
+      // $phpversion is safe and output of l() is safe, so this value is safe.
+      'value' => new SafeMarkup(($phase == 'runtime') ? $phpversion . ' (' . l(t('more information'), 'admin/reports/status/php') . ')' : $phpversion),
     );
   }
   else {
@@ -318,7 +320,8 @@ function system_requirements($phase) {
       'title' => t('Cron maintenance tasks'),
       'severity' => $severity,
       'value' => $summary,
-      'description' => $description
+      // @todo Needs to preserve safe markup.
+      'description' => new SafeMarkup($description),
     );
   }
   if ($phase != 'install') {
diff --git a/core/modules/system/templates/block--system-branding-block.html.twig b/core/modules/system/templates/block--system-branding-block.html.twig
index 2a12c7a..4cf0f1a 100644
--- a/core/modules/system/templates/block--system-branding-block.html.twig
+++ b/core/modules/system/templates/block--system-branding-block.html.twig
@@ -23,7 +23,7 @@
   {% endif %}
   {% if site_name %}
     <div class="site-name">
-      <a href="{{ url('<front>') }}" title="{{ 'Home'|t }}" rel="home">{{ site_name|e }}</a>
+      <a href="{{ url('<front>') }}" title="{{ 'Home'|t }}" rel="home">{{ site_name }}</a>
     </div>
   {% endif %}
   {% if site_slogan %}
diff --git a/core/modules/system/templates/datetime.html.twig b/core/modules/system/templates/datetime.html.twig
index 25ef788..183b834 100644
--- a/core/modules/system/templates/datetime.html.twig
+++ b/core/modules/system/templates/datetime.html.twig
@@ -25,5 +25,4 @@
  * @see http://www.w3.org/TR/html5-author/the-time-element.html#attr-time-datetime
  */
 #}
-{# @todo Revisit once http://drupal.org/node/1825952 is resolved. #}
-<time{{ attributes }}>{{ html ? text|raw : text|escape }}</time>
+<time{{ attributes }}>{{ html ? text|raw : text }}</time>
diff --git a/core/modules/user/user.module b/core/modules/user/user.module
index 3fe1f1c..b6a2d4b 100644
--- a/core/modules/user/user.module
+++ b/core/modules/user/user.module
@@ -1,6 +1,7 @@
 <?php
 
 use Drupal\Component\Utility\Crypt;
+use Drupal\Component\Utility\String;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Session\AccountInterface;
@@ -679,7 +680,10 @@ function theme_username($variables) {
     // We have a link path, so we should generate a link using l().
     // Additional classes may be added as array elements like
     // $variables['link_options']['attributes']['class'][] = 'myclass';
-    $output = l($variables['name'] . $variables['extra'], $variables['link_path'], $variables['link_options']);
+    if ($variables['extra']) {
+     $variables['name'] = String::concat($variables['name'], $variables['extra']);
+    }
+    $output = l($variables['name'], $variables['link_path'], $variables['link_options']);
   }
   else {
     // Modules may have added important attributes so they must be included
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/HandlerBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/HandlerBase.php
index cbff847..da8ddb1 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/HandlerBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/HandlerBase.php
@@ -236,7 +236,7 @@ public function getField($field = NULL) {
    * @param $type
    *   The type of sanitization needed. If not provided, String::checkPlain() is used.
    *
-   * @return string
+   * @return \Drupal\Core\Template\SafeMarkup
    *   Returns the safe value.
    */
   public function sanitizeValue($value, $type = NULL) {
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/field/FieldPluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/field/FieldPluginBase.php
index 482a032..a3dd7be 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/field/FieldPluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/field/FieldPluginBase.php
@@ -11,6 +11,7 @@
 use Drupal\Component\Utility\String;
 use Drupal\Component\Utility\UrlHelper;
 use Drupal\Component\Utility\Xss;
+use Drupal\Core\Template\SafeMarkup;
 use Drupal\views\Plugin\views\HandlerBase;
 use Drupal\views\Plugin\views\display\DisplayPluginBase;
 use Drupal\views\ResultRow;
@@ -1172,6 +1173,10 @@ public function advancedRender(ResultRow $values) {
         $this->last_render = $this->renderText($alter);
       }
     }
+    // @TODO: this is very dicey!
+    if ($this->last_render && is_string($this->last_render)) {
+      $this->last_render = new SafeMarkup($this->last_render);
+    }
 
     return $this->last_render;
   }
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/filter/FilterPluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/filter/FilterPluginBase.php
index 902b543..42b8d0b 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/filter/FilterPluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/filter/FilterPluginBase.php
@@ -1159,8 +1159,7 @@ protected function prepareFilterSelectOptions(&$options) {
         $this->prepareFilterSelectOptions($options[$value]);
       }
       // FAPI has some special value to allow hierarchy.
-      // @see _form_options_flatten
-      elseif (is_object($label)) {
+      elseif (is_object($label) && isset($options[$value]->option)) {
         $this->prepareFilterSelectOptions($options[$value]->option);
       }
       else {
diff --git a/core/modules/views/views.module b/core/modules/views/views.module
index 216bc93..fffa64b 100644
--- a/core/modules/views/views.module
+++ b/core/modules/views/views.module
@@ -903,7 +903,7 @@ function views_pre_render_views_form_views_form($element) {
   }
 
   // Apply substitutions to the rendered output.
-  $element['output']['#markup'] = str_replace($search, $replace, $element['output']['#markup']);
+  $element['output']['#markup'] = String::strReplace($search, $replace, $element['output']['#markup']);
 
   // Sort, render and add remaining form fields.
   $children = Element::children($element, TRUE);
diff --git a/core/modules/views/views.theme.inc b/core/modules/views/views.theme.inc
index ae967bd..3678423 100644
--- a/core/modules/views/views.theme.inc
+++ b/core/modules/views/views.theme.inc
@@ -9,6 +9,7 @@
 use Drupal\Component\Utility\Xss;
 use Drupal\Core\Language\Language;
 use Drupal\Core\Template\Attribute;
+use Drupal\Core\Template\SafeMarkup;
 use Drupal\views\Form\ViewsForm;
 use Drupal\views\ViewExecutable;
 
@@ -525,6 +526,7 @@ function template_preprocess_views_view_table(&$variables) {
 
     // Render the header labels.
     if ($field == $column && empty($fields[$field]->options['exclude'])) {
+      $safe = TRUE;
       $label = String::checkPlain(!empty($fields[$field]) ? $fields[$field]->label() : '');
       if (empty($options['info'][$field]['sortable']) || !$fields[$field]->clickSortable()) {
         $variables['header'][$field]['content'] = $label;
@@ -542,7 +544,10 @@ function template_preprocess_views_view_table(&$variables) {
             '#theme' => 'tablesort_indicator',
             '#style' => $initial,
           );
-          $label .= drupal_render($tablesort_indicator);
+          $markup = drupal_render($tablesort_indicator);
+          // $label is safe.
+          $safe = $markup instanceof SafeMarkup;
+          $label .= $markup;
         }
 
         $query['order'] = $field;
@@ -552,7 +557,7 @@ function template_preprocess_views_view_table(&$variables) {
           'attributes' => array('title' => $title),
           'query' => $query,
         );
-        $variables['header'][$field]['content'] = l($label, current_path(), $link_options);
+        $variables['header'][$field]['content'] = l($safe ? new SafeMarkup($label) : $label, current_path(), $link_options);
       }
 
       // Set up the header label class.
diff --git a/core/modules/views_ui/lib/Drupal/views_ui/Controller/ViewsUIController.php b/core/modules/views_ui/lib/Drupal/views_ui/Controller/ViewsUIController.php
index a09b063..d4673be 100644
--- a/core/modules/views_ui/lib/Drupal/views_ui/Controller/ViewsUIController.php
+++ b/core/modules/views_ui/lib/Drupal/views_ui/Controller/ViewsUIController.php
@@ -92,7 +92,7 @@ public function reportFields() {
       foreach ($views as $view) {
         $rows[$field_name]['data'][1][] = $this->l($view, 'views_ui.edit', array('view' => $view));
       }
-      $rows[$field_name]['data'][1] = implode(', ', $rows[$field_name]['data'][1]);
+      $rows[$field_name]['data'][1] = String::implode(', ', $rows[$field_name]['data'][1]);
     }
 
     // Sort rows by field name.
@@ -120,7 +120,7 @@ public function reportPlugins() {
       foreach ($row['views'] as $row_name => $view) {
         $row['views'][$row_name] = $this->l($view, 'views_ui.edit', array('view' => $view));
       }
-      $row['views'] = implode(', ', $row['views']);
+      $row['views'] = String::implode(', ', $row['views']);
     }
 
     // Sort rows by field name.
diff --git a/core/modules/views_ui/views_ui.theme.inc b/core/modules/views_ui/views_ui.theme.inc
index 430f14a..dea00af 100644
--- a/core/modules/views_ui/views_ui.theme.inc
+++ b/core/modules/views_ui/views_ui.theme.inc
@@ -6,7 +6,6 @@
  */
 
 use Drupal\Core\Render\Element;
-use Drupal\Core\Template\Attribute;
 
 /**
  * Prepares variables for Views UI display tab setting templates.
diff --git a/core/themes/bartik/templates/block--system-branding-block.html.twig b/core/themes/bartik/templates/block--system-branding-block.html.twig
index 5917f58..f6147a6 100644
--- a/core/themes/bartik/templates/block--system-branding-block.html.twig
+++ b/core/themes/bartik/templates/block--system-branding-block.html.twig
@@ -23,7 +23,7 @@
     <div class="site-branding-text">
       {% if site_name %}
         <strong class="site-name">
-          <a href="{{ url('<front>') }}" title="{{ 'Home'|t }}" rel="home">{{ site_name|e }}</a>
+          <a href="{{ url('<front>') }}" title="{{ 'Home'|t }}" rel="home">{{ site_name }}</a>
         </strong>
       {% endif %}
       {% if site_slogan %}
diff --git a/core/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index 1595bf8..47376dc 100644
--- a/core/themes/engines/twig/twig.engine
+++ b/core/themes/engines/twig/twig.engine
@@ -6,6 +6,7 @@
  */
 
 use Drupal\Core\Extension\Extension;
+use Drupal\Core\Template\SafeMarkup;
 
 /**
  * Implements hook_theme().
@@ -93,7 +94,7 @@ function twig_render_template($template_file, $variables) {
     $output['debug_info']   .= "\n<!-- BEGIN OUTPUT from '{$template_file}' -->\n";
     $output['debug_suffix'] .= "\n<!-- END OUTPUT from '{$template_file}' -->\n\n";
   }
-  return implode('', $output);
+  return new SafeMarkup(implode('', $output));
 }
 
 /**
diff --git a/core/vendor/twig/twig/lib/Twig/Extension/Core.php b/core/vendor/twig/twig/lib/Twig/Extension/Core.php
index 4e80c67..cdb5874 100644
--- a/core/vendor/twig/twig/lib/Twig/Extension/Core.php
+++ b/core/vendor/twig/twig/lib/Twig/Extension/Core.php
@@ -922,7 +922,11 @@ function twig_escape_filter(Twig_Environment $env, $string, $strategy = 'html',
 
     if (!is_string($string)) {
         if (is_object($string) && method_exists($string, '__toString')) {
-            $string = (string) $string;
+            // @todo Move to twig engine.
+            $string = $string->__toString();
+            if ($autoescape && $string instanceof Twig_Markup) {
+              return $string;
+            }
         } else {
             return $string;
         }