core/core.services.yml                             |  2 +-
 .../Core/Display/Annotation/DisplayVariant.php     |  2 +-
 .../EventSubscriber/MainContentViewSubscriber.php  |  9 ++----
 .../Drupal/Core/Render/BareHtmlPageRenderer.php    |  4 ++-
 .../Core/Render/BareHtmlPageRendererInterface.php  | 14 +++++---
 .../Core/Render/MainContent/DialogRenderer.php     |  2 +-
 .../Core/Render/MainContent/HtmlRenderer.php       | 37 +++++++++++++++-------
 .../MainContent/MainContentRenderersPass.php       |  2 +-
 .../Render}/PageDisplayVariantSelectionEvent.php   | 30 +++++++++++++++---
 .../Plugin/DisplayVariant/SimplePageVariant.php    |  4 +--
 .../Drupal/Core/Render/RenderEvents.php}           | 13 ++++----
 core/modules/block/block.services.yml              |  1 -
 .../BlockPageDisplayVariantSubscriber.php          | 28 +++-------------
 .../system/src/Tests/Common/PageRenderTest.php     |  8 +++--
 core/modules/system/theme.api.php                  |  4 +--
 15 files changed, 92 insertions(+), 68 deletions(-)

diff --git a/core/core.services.yml b/core/core.services.yml
index 52050cd..57f3cc3 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -659,7 +659,7 @@ services:
       - { name: event_subscriber }
   main_content_renderer.html:
     class: Drupal\Core\Render\MainContent\HtmlRenderer
-    arguments: ['@title_resolver', '@plugin.manager.display_variant', '@event_dispatcher']
+    arguments: ['@title_resolver', '@plugin.manager.display_variant', '@event_dispatcher', '@module_handler']
     tags:
       - { name: render.main_content_renderer, format: html }
   main_content_renderer.ajax:
diff --git a/core/lib/Drupal/Core/Display/Annotation/DisplayVariant.php b/core/lib/Drupal/Core/Display/Annotation/DisplayVariant.php
index df1b52a..53d58e3 100644
--- a/core/lib/Drupal/Core/Display/Annotation/DisplayVariant.php
+++ b/core/lib/Drupal/Core/Display/Annotation/DisplayVariant.php
@@ -26,7 +26,7 @@
  * Plugin namespace: Plugin\DisplayVariant
  *
  * For working examples, see
- * - \Drupal\system\Plugin\DisplayVariant\SimplePageVariant
+ * - \Drupal\Core\Render\Plugin\DisplayVariant\SimplePageVariant
  * - \Drupal\block\Plugin\DisplayVariant\BlockPageVariant
  * - \Drupal\block\Plugin\DisplayVariant\DemoBlockPageVariant
  *
diff --git a/core/lib/Drupal/Core/EventSubscriber/MainContentViewSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/MainContentViewSubscriber.php
index 0c386bf..75c3028 100644
--- a/core/lib/Drupal/Core/EventSubscriber/MainContentViewSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/MainContentViewSubscriber.php
@@ -57,7 +57,7 @@ class MainContentViewSubscriber implements EventSubscriberInterface {
    * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
    *   The current route match.
    * @param array $main_content_renderers
-   *   The available main content renderer services, keyed per format.
+   *   The available main content renderer service IDs, keyed by format.
    */
   public function __construct(ClassResolverInterface $class_resolver, RouteMatchInterface $route_match, array $main_content_renderers) {
     $this->classResolver = $class_resolver;
@@ -95,13 +95,10 @@ public function onViewRenderArray(GetResponseForControllerResultEvent $event) {
   }
 
   /**
-   * Registers the methods in this class that should be listeners.
-   *
-   * @return array
-   *   An array of event listener definitions.
+   * {@inheritdoc}
    */
   static function getSubscribedEvents() {
-    $events[KernelEvents::VIEW][] = array('onViewRenderArray', 30);
+    $events[KernelEvents::VIEW][] = ['onViewRenderArray'];
 
     return $events;
   }
diff --git a/core/lib/Drupal/Core/Render/BareHtmlPageRenderer.php b/core/lib/Drupal/Core/Render/BareHtmlPageRenderer.php
index cd5adaa..a8904e6 100644
--- a/core/lib/Drupal/Core/Render/BareHtmlPageRenderer.php
+++ b/core/lib/Drupal/Core/Render/BareHtmlPageRenderer.php
@@ -70,7 +70,9 @@ protected function renderBarePage(array $content, $title, array $page_additions,
     ];
 
     // We must first render the contents of the html.html.twig template, see
-    // \Drupal\Core\Render\MainContent\HtmlRenderer::renderPage() for details.
+    // \Drupal\Core\Render\MainContent\HtmlRenderer::renderResponse() for more
+    // information about this; the exact same pattern is used there and
+    // explained in detail there.
     drupal_render_root($html['page']);
     return drupal_render($html);
   }
diff --git a/core/lib/Drupal/Core/Render/BareHtmlPageRendererInterface.php b/core/lib/Drupal/Core/Render/BareHtmlPageRendererInterface.php
index 4bba7c8..7450199 100644
--- a/core/lib/Drupal/Core/Render/BareHtmlPageRendererInterface.php
+++ b/core/lib/Drupal/Core/Render/BareHtmlPageRendererInterface.php
@@ -25,16 +25,22 @@
  * - exception handlers
  *
  * i.e. use this when rendering HTML pages in limited environments. Otherwise,
- * use a @code _content @endcode route, thiswill cause a main content controller
- * (\Drupal\Core\ControllerMainContentControllerInterface) to be used, and in
- * case of a HTML request that will be
+ * use a @code _content @endcode route, this will cause a main content renderer
+ * (\Drupal\Core\Render\MainContent\MainContentRendererInterface) to be used,
+ * and in case of a HTML request that will be
  * \Drupal\Core\Render\MainContent\HtmlRenderer.
  *
+ * In fact, this is not only *typically* used in a limited environment, it even
+ * *must* be used in a limited environment: when using the bare HTML page
+ * renderer, use as little state/additional services as possible, because the
+ * same safeguards aren't present (precisely because this is intended to be used
+ * in a limited environment).
+ *
  * Currently, there are two types of bare pages available:
  * 1. install (hook_preprocess_install_page(), install-page.html.twig)
  * 2. maintenance (hook_preprocess_maintenance_page(), maintenance-page.html.twig)
  *
- * @see \Drupal\Core\Controller\HtmlController
+ * @see \Drupal\Core\Render\MainContent\HtmlRenderer
  */
 interface BareHtmlPageRendererInterface {
 
diff --git a/core/lib/Drupal/Core/Render/MainContent/DialogRenderer.php b/core/lib/Drupal/Core/Render/MainContent/DialogRenderer.php
index 24462cb..adb550a 100644
--- a/core/lib/Drupal/Core/Render/MainContent/DialogRenderer.php
+++ b/core/lib/Drupal/Core/Render/MainContent/DialogRenderer.php
@@ -21,7 +21,7 @@ class DialogRenderer {
   /**
    * The title resolver.
    *
-   * @var \Drupal\Core\Controller\TitleResolver
+   * @var \Drupal\Core\Controller\TitleResolverInterface
    */
   protected $titleResolver;
 
diff --git a/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php b/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
index 47e33bb..3783c5d 100644
--- a/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
+++ b/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
@@ -12,9 +12,10 @@
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Controller\TitleResolverInterface;
 use Drupal\Core\Display\PageVariantInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Render\PageDisplayVariantSelectionEvent;
+use Drupal\Core\Render\RenderEvents;
 use Drupal\Core\Routing\RouteMatchInterface;
-use Drupal\system\Event\PageDisplayVariantSelectionEvent;
-use Drupal\system\Event\SystemEvents;
 use Symfony\Component\DependencyInjection\ContainerAwareTrait;
 use Symfony\Component\EventDispatcher\EventDispatcherInterface;
 use Symfony\Component\HttpFoundation\Request;
@@ -47,6 +48,13 @@ class HtmlRenderer {
   protected $eventDispatcher;
 
   /**
+   * The module handler.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   */
+  protected $moduleHandler;
+
+  /**
    * Constructs a new HtmlRenderer.
    *
    * @param \Drupal\Core\Controller\TitleResolverInterface $title_resolver
@@ -55,11 +63,14 @@ class HtmlRenderer {
    *   The display variant manager.
    * @param \Symfony\Component\EventDispatcher\EventDispatcherInterface $event_dispatcher
    *   The event dispatcher.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler.
    */
-  public function __construct(TitleResolverInterface $title_resolver, PluginManagerInterface $display_variant_manager, EventDispatcherInterface $event_dispatcher) {
+  public function __construct(TitleResolverInterface $title_resolver, PluginManagerInterface $display_variant_manager, EventDispatcherInterface $event_dispatcher, ModuleHandlerInterface $module_handler) {
     $this->titleResolver = $title_resolver;
     $this->displayVariantManager = $display_variant_manager;
     $this->eventDispatcher = $event_dispatcher;
+    $this->moduleHandler = $module_handler;
   }
 
   /**
@@ -91,8 +102,8 @@ protected function prepare(array $main_content, Request $request, RouteMatchInte
     else {
       // Select the page display variant to be used to render this main content,
       // default to the built-in "simple page".
-      $event = new PageDisplayVariantSelectionEvent('simple_page');
-      $this->eventDispatcher->dispatch(SystemEvents::SELECT_PAGE_DISPLAY_VARIANT, $event);
+      $event = new PageDisplayVariantSelectionEvent('simple_page', $route_match);
+      $this->eventDispatcher->dispatch(RenderEvents::SELECT_PAGE_DISPLAY_VARIANT, $event);
       $variant_id = $event->getPluginId();
 
       // We must render the main content now already, because it might provide a
@@ -219,13 +230,15 @@ public function renderResponse(array $main_content, Request $request, RouteMatch
    *
    * @throws \LogicException
    *
+   * @internal
+   *
    * @see hook_page_attachments()
    * @see hook_page_attachments_alter()
    */
-  public static function invokePageAttachmentHooks(array &$page) {
+  public function invokePageAttachmentHooks(array &$page) {
     // Modules can add attachments.
     $attachments = [];
-    foreach (\Drupal::moduleHandler()->getImplementations('page_attachments') as $module) {
+    foreach ($this->moduleHandler->getImplementations('page_attachments') as $module) {
       $function = $module . '_page_attachments';
       $function($attachments);
     }
@@ -234,7 +247,7 @@ public static function invokePageAttachmentHooks(array &$page) {
     }
 
     // Modules and themes can alter page attachments.
-    \Drupal::moduleHandler()->alter('page_attachments', $attachments);
+    $this->moduleHandler->alter('page_attachments', $attachments);
     \Drupal::theme()->alter('page_attachments', $attachments);
     if (array_diff(array_keys($attachments), ['#attached', '#post_render_cache']) !== []) {
       throw new \LogicException('Only #attached and #post_render_cache may be set in hook_page_attachments_alter().');
@@ -257,19 +270,21 @@ public static function invokePageAttachmentHooks(array &$page) {
    *
    * @throws \LogicException
    *
+   * @internal
+   *
    * @see hook_page_top()
    * @see hook_page_bottom()
    * @see html.html.twig
    */
-  public static function buildPageTopAndBottom(array &$html) {
+  public function buildPageTopAndBottom(array &$html) {
     // Modules can add render arrays to the top and bottom of the page.
     $page_top = [];
     $page_bottom = [];
-    foreach (\Drupal::moduleHandler()->getImplementations('page_top') as $module) {
+    foreach ($this->moduleHandler->getImplementations('page_top') as $module) {
       $function = $module . '_page_top';
       $function($page_top);
     }
-    foreach (\Drupal::moduleHandler()->getImplementations('page_bottom') as $module) {
+    foreach ($this->moduleHandler->getImplementations('page_bottom') as $module) {
       $function = $module . '_page_bottom';
       $function($page_bottom);
     }
diff --git a/core/lib/Drupal/Core/Render/MainContent/MainContentRenderersPass.php b/core/lib/Drupal/Core/Render/MainContent/MainContentRenderersPass.php
index dbb72be..08b4665 100644
--- a/core/lib/Drupal/Core/Render/MainContent/MainContentRenderersPass.php
+++ b/core/lib/Drupal/Core/Render/MainContent/MainContentRenderersPass.php
@@ -18,7 +18,7 @@ class MainContentRenderersPass implements CompilerPassInterface {
   /**
    * {@inheritdoc}
    *
-   * Collects the available main content renderer services into the
+   * Collects the available main content renderer service IDs into the
    * main_content_renderers parameter, keyed by format.
    */
   public function process(ContainerBuilder $container) {
diff --git a/core/modules/system/src/Event/PageDisplayVariantSelectionEvent.php b/core/lib/Drupal/Core/Render/PageDisplayVariantSelectionEvent.php
similarity index 52%
rename from core/modules/system/src/Event/PageDisplayVariantSelectionEvent.php
rename to core/lib/Drupal/Core/Render/PageDisplayVariantSelectionEvent.php
index 9177a18..17771bd 100644
--- a/core/modules/system/src/Event/PageDisplayVariantSelectionEvent.php
+++ b/core/lib/Drupal/Core/Render/PageDisplayVariantSelectionEvent.php
@@ -2,17 +2,19 @@
 
 /**
  * @file
- * Contains \Drupal\system\Event\PageDisplayVariantSelectionEvent.
+ * Contains \Drupal\Core\Render\PageDisplayVariantSelectionEvent.
  */
 
-namespace Drupal\system\Event;
+namespace Drupal\Core\Render;
 
+use Drupal\Core\Routing\RouteMatchInterface;
 use Symfony\Component\EventDispatcher\Event;
 
 /**
  * Event fired when rendering main content, to select a page display variant.
  *
- * @see \Drupal\system\Event\SystemEvents::SELECT_PAGE_DISPLAY_VARIANT
+ * @see \Drupal\Core\Render\RenderEvents::SELECT_PAGE_DISPLAY_VARIANT
+ * @see \Drupal\Core\Render\MainContent\HtmlRenderer
  */
 class PageDisplayVariantSelectionEvent extends Event {
 
@@ -24,13 +26,23 @@ class PageDisplayVariantSelectionEvent extends Event {
   protected $pluginId;
 
   /**
+   * The current route match.
+   *
+   * @var \Drupal\Core\Routing\RouteMatchInterface
+   */
+  protected $routeMatch;
+
+  /**
    * Constructs the page display variant plugin selection event.
    *
    * @param string
    *   The ID of the page display variant plugin to use by default.
+   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
+   *   The current route match, for context.
    */
-  public function __construct($plugin_id) {
+  public function __construct($plugin_id, RouteMatchInterface $route_match) {
     $this->pluginId = $plugin_id;
+    $this->routeMatch = $route_match;
   }
 
   /**
@@ -52,4 +64,14 @@ public function getPluginId() {
     return $this->pluginId;
   }
 
+  /**
+   * Gets the current route match.
+   *
+   * @return \Drupal\Core\Routing\RouteMatchInterface
+   *   The current route match, for context.
+   */
+  public function getRouteMatch() {
+    return $this->routeMatch;
+  }
+
 }
diff --git a/core/modules/system/src/Plugin/DisplayVariant/SimplePageVariant.php b/core/lib/Drupal/Core/Render/Plugin/DisplayVariant/SimplePageVariant.php
similarity index 85%
rename from core/modules/system/src/Plugin/DisplayVariant/SimplePageVariant.php
rename to core/lib/Drupal/Core/Render/Plugin/DisplayVariant/SimplePageVariant.php
index afe298a..7772875 100644
--- a/core/modules/system/src/Plugin/DisplayVariant/SimplePageVariant.php
+++ b/core/lib/Drupal/Core/Render/Plugin/DisplayVariant/SimplePageVariant.php
@@ -2,10 +2,10 @@
 
 /**
  * @file
- * Contains \Drupal\system\Plugin\DisplayVariant\SimplePageVariant.
+ * Contains \Drupal\Core\Render\Plugin\DisplayVariant\SimplePageVariant.
  */
 
-namespace Drupal\system\Plugin\DisplayVariant;
+namespace Drupal\Core\Render\Plugin\DisplayVariant;
 
 use Drupal\Core\Display\PageVariantInterface;
 use Drupal\Core\Display\VariantBase;
diff --git a/core/modules/system/src/Event/SystemEvents.php b/core/lib/Drupal/Core/Render/RenderEvents.php
similarity index 26%
rename from core/modules/system/src/Event/SystemEvents.php
rename to core/lib/Drupal/Core/Render/RenderEvents.php
index a99eb59..8b0fa55 100644
--- a/core/modules/system/src/Event/SystemEvents.php
+++ b/core/lib/Drupal/Core/Render/RenderEvents.php
@@ -2,22 +2,21 @@
 
 /**
  * @file
- * Contains \Drupal\system\Event\SystemEvents.
+ * Contains \Drupal\Core\Render\RenderEvents.
  */
 
-namespace Drupal\system\Event;
+namespace Drupal\Core\Render;
 
 /**
- * Defines events for the base system.
+ * Defines events for the render system.
  */
-final class SystemEvents {
+final class RenderEvents {
 
   /**
    * Name of the event when selecting a page display variant to use.
    *
-   * @see \Drupal\Core\Block\BlockBase::getConditionContexts()
-   * @see \Drupal\system\Event\PageDisplayVariantSelectionEvent
+   * @see \Drupal\Core\Render\PageDisplayVariantSelectionEvent
    */
-  const SELECT_PAGE_DISPLAY_VARIANT= 'system.page_display_variant.select';
+  const SELECT_PAGE_DISPLAY_VARIANT = 'render.page_display_variant.select';
 
 }
diff --git a/core/modules/block/block.services.yml b/core/modules/block/block.services.yml
index 9c7be32..601ff51 100644
--- a/core/modules/block/block.services.yml
+++ b/core/modules/block/block.services.yml
@@ -5,7 +5,6 @@ services:
       - { name: theme_negotiator, priority: 1000 }
   block.page_display_variant_subscriber:
     class: Drupal\block\EventSubscriber\BlockPageDisplayVariantSubscriber
-    arguments: ['@current_route_match']
     tags:
       - { name: event_subscriber }
   block.current_user_context:
diff --git a/core/modules/block/src/EventSubscriber/BlockPageDisplayVariantSubscriber.php b/core/modules/block/src/EventSubscriber/BlockPageDisplayVariantSubscriber.php
index b6127fb..1a13d26 100644
--- a/core/modules/block/src/EventSubscriber/BlockPageDisplayVariantSubscriber.php
+++ b/core/modules/block/src/EventSubscriber/BlockPageDisplayVariantSubscriber.php
@@ -7,10 +7,9 @@
 
 namespace Drupal\block\EventSubscriber;
 
-use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\StringTranslation\StringTranslationTrait;
-use Drupal\system\Event\PageDisplayVariantSelectionEvent;
-use Drupal\system\Event\SystemEvents;
+use Drupal\Core\Render\PageDisplayVariantSelectionEvent;
+use Drupal\Core\Render\RenderEvents;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 
 /**
@@ -22,30 +21,13 @@
 class BlockPageDisplayVariantSubscriber implements EventSubscriberInterface {
 
   /**
-   * The current route match.
-   *
-   * @var \Drupal\Core\Routing\RouteMatchInterface
-   */
-  protected $routeMatch;
-
-  /**
-   * Constructs a BlockPageDisplayVariantSubscriber object.
-   *
-   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
-   *   The current route match.
-   */
-  public function __construct(RouteMatchInterface $route_match) {
-    $this->routeMatch = $route_match;
-  }
-
-  /**
    * Selects the block page display variant.
    *
-   * @param \Drupal\system\Event\PageDisplayVariantSelectionEvent $event
+   * @param \Drupal\Core\Render\PageDisplayVariantSelectionEvent $event
    *   The event to process.
    */
   public function onSelectPageDisplayVariant(PageDisplayVariantSelectionEvent $event) {
-    if ($this->routeMatch->getRouteName() != 'block.admin_demo') {
+    if ($event->getRouteMatch()->getRouteName() != 'block.admin_demo') {
       $event->setPluginId('block_page');
     }
     else {
@@ -57,7 +39,7 @@ public function onSelectPageDisplayVariant(PageDisplayVariantSelectionEvent $eve
    * {@inheritdoc}
    */
   static function getSubscribedEvents() {
-    $events[SystemEvents::SELECT_PAGE_DISPLAY_VARIANT][] = array('onSelectPageDisplayVariant');
+    $events[RenderEvents::SELECT_PAGE_DISPLAY_VARIANT][] = array('onSelectPageDisplayVariant');
     return $events;
   }
 
diff --git a/core/modules/system/src/Tests/Common/PageRenderTest.php b/core/modules/system/src/Tests/Common/PageRenderTest.php
index ad8a087..e660448 100644
--- a/core/modules/system/src/Tests/Common/PageRenderTest.php
+++ b/core/modules/system/src/Tests/Common/PageRenderTest.php
@@ -60,16 +60,18 @@ function testHookPageAttachmentsAlter() {
    *   The page render hook to assert expected exceptions for.
    */
   function assertPageRenderHookExceptions($module, $hook) {
+    $html_renderer = \Drupal::getContainer()->get('main_content_renderer.html');
+
     // Assert a valid hook implementation doesn't trigger an exception.
     $page = [];
-    HtmlRenderer::invokePageAttachmentHooks($page);
+    $html_renderer->invokePageAttachmentHooks($page);
 
     // Assert an invalid hook implementation doesn't trigger an exception.
     \Drupal::state()->set($module . '.' . $hook . '.descendant_attached', TRUE);
     $assertion = $hook . '() implementation that sets #attached on a descendant triggers an exception';
     $page = [];
     try {
-      HtmlRenderer::invokePageAttachmentHooks($page);
+      $html_renderer->invokePageAttachmentHooks($page);
       $this->error($assertion);
     }
     catch (\LogicException $e) {
@@ -83,7 +85,7 @@ function assertPageRenderHookExceptions($module, $hook) {
     $assertion = $hook . '() implementation that sets a child render array triggers an exception';
     $page = [];
     try {
-      HtmlRenderer::invokePageAttachmentHooks($page);
+      $html_renderer->invokePageAttachmentHooks($page);
       $this->error($assertion);
     }
     catch (\LogicException $e) {
diff --git a/core/modules/system/theme.api.php b/core/modules/system/theme.api.php
index 37b769e..b7fe983 100644
--- a/core/modules/system/theme.api.php
+++ b/core/modules/system/theme.api.php
@@ -352,9 +352,9 @@
  * 1. \Drupal\Core\Render\MainContent\HtmlRenderer::prepare() takes the main
  *    content render array; if it already is #type 'page', then most of the work
  *    it must do is already done. In the other case, we need to build that #type
- *    'page' render array still. The SystemEvents::SELECT_PAGE_DISPLAY_VARIANT
+ *    'page' render array still. The RenderEvents::SELECT_PAGE_DISPLAY_VARIANT
  *    event is dispatched, to select a page display variant. By default,
- *    \Drupal\system\Plugin\DisplayVariant\SimplePageVariant is used, which
+ *    \Drupal\Core\Render\Plugin\DisplayVariant\SimplePageVariant is used, which
  *    doesn't apply any decorations. But, when Block module is enabled,
  *    \Drupal\block\Plugin\DisplayVariant\BlockPageVariant is used, which allows
  *    the site builder to place blocks in any of the page regions, and hence