diff --git a/core/core.services.yml b/core/core.services.yml
index ff9ee96..99ae337 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -995,7 +995,7 @@ services:
     class: Zend\Feed\Writer\Extension\WellFormedWeb\Renderer\Entry
   theme.manager:
     class: Drupal\Core\Theme\ThemeManager
-    arguments: ['@theme.registry', '@theme.negotiator', '@theme.initialization', '@request_stack', '@module_handler']
+    arguments: ['@app.root', '@theme.registry', '@theme.negotiator', '@theme.initialization', '@request_stack', '@module_handler']
   theme.initialization:
     class: Drupal\Core\Theme\ThemeInitialization
     arguments: ['@app.root', '@theme_handler', '@state']
diff --git a/core/includes/common.inc b/core/includes/common.inc
index 0397ee5..beb28b7 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -2383,7 +2383,7 @@ function drupal_pre_render_links($element) {
  * @deprecated as of Drupal 8.0.x. Use the 'renderer' service instead.
  */
 function drupal_render_root(&$elements) {
-  return \Drupal::service('renderer')->render($elements, TRUE);
+  return \Drupal::service('renderer')->renderRoot($elements);
 }
 
 /**
diff --git a/core/lib/Drupal/Core/Render/Renderer.php b/core/lib/Drupal/Core/Render/Renderer.php
index a2d4e5a..5fb494e 100644
--- a/core/lib/Drupal/Core/Render/Renderer.php
+++ b/core/lib/Drupal/Core/Render/Renderer.php
@@ -14,7 +14,7 @@
 use Drupal\Component\Utility\NestedArray;
 
 /**
- * Converts a render array into a string.
+ * Turns a render array into a HTML string.
  */
 class Renderer implements RendererInterface {
 
@@ -58,6 +58,13 @@ public function __construct(ControllerResolverInterface $controller_resolver, Th
   /**
    * {@inheritdoc}
    */
+  public function renderRoot(&$elements) {
+    return $this->render($elements, TRUE);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function render(&$elements, $is_root_call = FALSE) {
     static $stack;
 
diff --git a/core/lib/Drupal/Core/Render/RendererInterface.php b/core/lib/Drupal/Core/Render/RendererInterface.php
index 897ab19..1eded65 100644
--- a/core/lib/Drupal/Core/Render/RendererInterface.php
+++ b/core/lib/Drupal/Core/Render/RendererInterface.php
@@ -13,6 +13,27 @@
 interface RendererInterface {
 
   /**
+   * Renders final HTML given a structured array tree.
+   *
+   * Calls ::render() in such a way that #post_render_cache callbacks are
+   * applied.
+   *
+   * Should therefore only be used in occasions where the final rendering is
+   * happening, just before sending a Response:
+   * - system internals that are responsible for rendering the final HTML
+   * - render arrays for non-HTML responses, such as feeds
+   *
+   * @param array $elements
+   *   The structured array describing the data to be rendered.
+   *
+   * @return string
+   *   The rendered HTML.
+   *
+   * @see ::render()
+   */
+  public function renderRoot(&$elements);
+
+  /**
    * Renders HTML given a structured array tree.
    *
    * Renderable arrays have two kinds of key/value pairs: properties and
@@ -119,24 +140,24 @@
    *     of overrides as the value in #theme_wrappers array.
    *     For example, if we have a render element as follows:
    * @code
-   *     array(
-   *       '#theme' => 'image',
-   *       '#attributes' => array('class' => array('foo')),
-   *       '#theme_wrappers' => array('container'),
-   *     );
+   *   array(
+   *     '#theme' => 'image',
+   *     '#attributes' => array('class' => array('foo')),
+   *     '#theme_wrappers' => array('container'),
+   *   );
    * @endcode
    *     and we need to pass the class 'bar' as an attribute for 'container', we
    *     can rewrite our element thus:
    * @code
-   *     array(
-   *       '#theme' => 'image',
-   *       '#attributes' => array('class' => array('foo')),
-   *       '#theme_wrappers' => array(
-   *         'container' => array(
-   *           '#attributes' => array('class' => array('bar')),
-   *         ),
+   *   array(
+   *     '#theme' => 'image',
+   *     '#attributes' => array('class' => array('foo')),
+   *     '#theme_wrappers' => array(
+   *       'container' => array(
+   *         '#attributes' => array('class' => array('bar')),
    *       ),
-   *     );
+   *     ),
+   *   );
    * @endcode
    *   - If this element has an array of #post_render functions defined, they
    *     are called sequentially to modify the rendered #children. Unlike
@@ -198,14 +219,14 @@
    *   The structured array describing the data to be rendered.
    * @param bool $is_root_call
    *   (Internal use only.) Whether this is a recursive call or not. See
-   *   drupal_render_root().
+   *   ::renderRoot().
    *
    * @return string
    *   The rendered HTML.
    *
    * @throws \LogicException
-   *   If a root call to drupal_render() does not result in an empty stack, this
-   *   indicates an erroneous drupal_render() root call (a root call within a
+   *   If a root call to ::render() does not result in an empty stack, this
+   *   indicates an erroneous ::render() root call (a root call within a
    *   root call, which makes no sense). Therefore, a logic exception is thrown.
    * @throws \Exception
    *   If a #pre_render callback throws an exception, it is caught to reset the
@@ -216,7 +237,7 @@
    * @see \Drupal\Core\Theme\ThemeManagerInterface::render()
    * @see drupal_process_states()
    * @see drupal_process_attached()
-   * @see drupal_render_root()
+   * @see ::renderRoot()
    */
   public function render(&$elements, $is_root_call = FALSE);
 
diff --git a/core/lib/Drupal/Core/Theme/ThemeManager.php b/core/lib/Drupal/Core/Theme/ThemeManager.php
index e58bfed..a100374 100644
--- a/core/lib/Drupal/Core/Theme/ThemeManager.php
+++ b/core/lib/Drupal/Core/Theme/ThemeManager.php
@@ -51,8 +51,17 @@ class ThemeManager implements ThemeManagerInterface {
   protected $requestStack;
 
   /**
+   * The app root.
+   *
+   * @var string
+   */
+  protected $root;
+
+  /**
    * Constructs a new ThemeManager object.
    *
+   * @param string $root
+   *   The app root.
    * @param \Drupal\Core\Theme\Registry $theme_registry
    *   The theme registry.
    * @param \Drupal\Core\Theme\ThemeNegotiatorInterface $theme_negotiator
@@ -61,8 +70,10 @@ class ThemeManager implements ThemeManagerInterface {
    *   The theme initialization.
    * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
    *   The request stack.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
    */
-  public function __construct(Registry $theme_registry, ThemeNegotiatorInterface $theme_negotiator, ThemeInitialization $theme_initialization, RequestStack $request_stack, ModuleHandlerInterface $module_handler) {
+  public function __construct($root, Registry $theme_registry, ThemeNegotiatorInterface $theme_negotiator, ThemeInitialization $theme_initialization, RequestStack $request_stack, ModuleHandlerInterface $module_handler) {
+    $this->root = $root;
     $this->themeNegotiator = $theme_negotiator;
     $this->themeRegistry = $theme_registry;
     $this->themeInitialization = $theme_initialization;
@@ -116,7 +127,7 @@ public function setActiveTheme(ActiveTheme $active_theme) {
   /**
    * Generates themed output (internal use only).
    *
-   * @see ::render for the full documentation.
+   * @see \Drupal\Core\Render\RendererInterface::render();
    */
   protected function theme($hook, $variables = array()) {
     static $default_attributes;
@@ -146,8 +157,9 @@ protected function theme($hook, $variables = array()) {
     // preprocess callbacks.
     $original_hook = $hook;
 
-    // If there's no implementation, check for more generic fallbacks. If there's
-    // still no implementation, log an error and return an empty string.
+    // If there's no implementation, check for more generic fallbacks.
+    // If there's still no implementation, log an error and return an empty
+    // string.
     if (!$theme_registry->has($hook)) {
       // Iteratively strip everything after the last '__' delimiter, until an
       // implementation is found.
@@ -165,7 +177,8 @@ protected function theme($hook, $variables = array()) {
         }
         // There is no theme implementation for the hook passed. Return FALSE so
         // the function calling _theme() can differentiate between a hook that
-        // exists and renders an empty string and a hook that is not implemented.
+        // exists and renders an empty string and a hook that is not
+        // implemented.
         return FALSE;
       }
     }
@@ -248,7 +261,7 @@ protected function theme($hook, $variables = array()) {
     // elsewhere.
     if (!empty($info['includes'])) {
       foreach ($info['includes'] as $include_file) {
-        include_once DRUPAL_ROOT . '/' . $include_file;
+        include_once $this->root . '/' . $include_file;
       }
     }
 
@@ -256,11 +269,11 @@ protected function theme($hook, $variables = array()) {
     if (isset($info['base hook'])) {
       $base_hook = $info['base hook'];
       $base_hook_info = $theme_registry->get($base_hook);
-      // Include files required by the base hook, since its variable preprocessors
-      // might reside there.
+      // Include files required by the base hook, since its variable
+      // preprocessors might reside there.
       if (!empty($base_hook_info['includes'])) {
         foreach ($base_hook_info['includes'] as $include_file) {
-          include_once DRUPAL_ROOT . '/' . $include_file;
+          include_once $this->root . '/' . $include_file;
         }
       }
       // Replace the preprocess functions with those from the base hook.
@@ -278,10 +291,10 @@ protected function theme($hook, $variables = array()) {
         }
       }
       // Allow theme preprocess functions to set $variables['#attached'] and use
-      // it like the #attached property on render arrays. In Drupal 8, this is the
-      // (only) officially supported method of attaching assets from preprocess
-      // functions. Assets attached here should be associated with the template
-      // that we're preprocessing variables for.
+      // it like the #attached property on render arrays. In Drupal 8, this is
+      // the (only) officially supported method of attaching assets from
+      // preprocess functions. Assets attached here should be associated with
+      // the template that we're preprocessing variables for.
       if (isset($variables['#attached'])) {
         $preprocess_attached = ['#attached' => $variables['#attached']];
         drupal_render($preprocess_attached);
@@ -299,7 +312,8 @@ protected function theme($hook, $variables = array()) {
       $render_function = 'twig_render_template';
       $extension = '.html.twig';
 
-      // The theme engine may use a different extension and a different renderer.
+      // The theme engine may use a different extension and a different
+      // renderer.
       $theme_engine = $active_theme->getEngine();
       if (isset($theme_engine)) {
         if ($info['type'] != 'module') {
@@ -314,15 +328,15 @@ protected function theme($hook, $variables = array()) {
       }
 
       // In some cases, a template implementation may not have had
-      // template_preprocess() run (for example, if the default implementation is
-      // a function, but a template overrides that default implementation). In
-      // these cases, a template should still be able to expect to have access to
-      // the variables provided by template_preprocess(), so we add them here if
-      // they don't already exist. We don't want the overhead of running
-      // template_preprocess() twice, so we use the 'directory' variable to
-      // determine if it has already run, which while not completely intuitive,
-      // is reasonably safe, and allows us to save on the overhead of adding some
-      // new variable to track that.
+      // template_preprocess() run (for example, if the default implementation
+      // is a function, but a template overrides that default implementation).
+      // In these cases, a template should still be able to expect to have
+      // access to the variables provided by template_preprocess(), so we add
+      // them here if they don't already exist. We don't want the overhead of
+      // running template_preprocess() twice, so we use the 'directory' variable
+      // to determine if it has already run, which while not completely
+      // intuitive, is reasonably safe, and allows us to save on the overhead of
+      // adding some new variable to track that.
       if (!isset($variables['directory'])) {
         $default_template_variables = array();
         template_preprocess($default_template_variables, $hook, $info);