core/includes/common.inc                           | 34 ++++++++++++++--------
 .../Core/Page/DefaultHtmlFragmentRenderer.php      |  8 +++--
 core/modules/block/block.install                   | 19 ------------
 .../content_translation/content_translation.module |  4 +--
 .../src/Tests/System/MainContentFallbackTest.php   |  8 +----
 core/modules/system/system.module                  | 15 ----------
 .../tests/modules/system_test/system_test.module   | 17 +++++------
 .../modules/system_test/system_test.routing.yml    |  8 -----
 .../tests/modules/theme_test/theme_test.module     |  6 ++--
 core/modules/system/theme.api.php                  | 20 +++++++++----
 core/modules/views/views.module                    | 20 ++++++++++---
 11 files changed, 73 insertions(+), 86 deletions(-)

diff --git a/core/includes/common.inc b/core/includes/common.inc
index eb11364..16b1194 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -945,8 +945,7 @@ function drupal_add_html_head_link($attributes, $header = FALSE) {
  * $options['preprocess'] should be only set to TRUE when a file is required for
  * all typical visitors and most pages of a site. It is critical that all
  * preprocessed files are added unconditionally on every page, even if the
- * files do not happen to be needed on a page. This is normally done by calling
- * _drupal_add_css() in a hook_page_build() implementation.
+ * files do not happen to be needed on a page.
  *
  * Non-preprocessed files should only be added to the page when they are
  * actually needed.
@@ -994,8 +993,8 @@ function drupal_add_html_head_link($attributes, $header = FALSE) {
  *     page of the website for users for whom it is present at all. This
  *     defaults to FALSE. It is set to TRUE for stylesheets added via module and
  *     theme .info.yml files. Modules that add stylesheets within
- *     hook_page_build() implementations, or from other code that ensures that
- *     the stylesheet is added to all website pages, should also set this flag
+ *     hook_page_attachments() implementations, or from other code that ensures
+ *     that the stylesheet is added to all website pages, should also set this flag
  *     to TRUE. All stylesheets within the same group that have the 'every_page'
  *     flag set to TRUE and do not have 'preprocess' set to FALSE are aggregated
  *     together into a single aggregate file, and that aggregate file can be
@@ -1395,8 +1394,7 @@ function drupal_clean_id_identifier($id) {
  * $options['preprocess'] should be only set to TRUE when a file is required for
  * all typical visitors and most pages of a site. It is critical that all
  * preprocessed files are added unconditionally on every page, even if the
- * files are not needed on a page. This is normally done by calling
- * _drupal_add_js() in a hook_page_build() implementation.
+ * files are not needed on a page.
  *
  * Non-preprocessed files should only be added to the page when they are
  * actually needed.
@@ -1439,9 +1437,9 @@ function drupal_clean_id_identifier($id) {
  *     page of the website for users for whom it is present at all. This
  *     defaults to FALSE. It is set to TRUE for JavaScript files that are added
  *     via module and theme .info.yml files. Modules that add JavaScript within
- *     hook_page_build() implementations, or from other code that ensures that
- *     the JavaScript is added to all website pages, should also set this flag
- *     to TRUE. All JavaScript files within the same group and that have the
+ *     hook_page_attachments() implementations, or from other code that ensures
+ *     that the JavaScript is added to all website pages, should also set this
+ *     flag to TRUE. All JavaScript files within the same group and that have the
  *     'every_page' flag set to TRUE and do not have 'preprocess' set to FALSE
  *     are aggregated together into a single aggregate file, and that aggregate
  *     file can be reused across a user's entire site visit, leading to faster
@@ -1740,13 +1738,13 @@ function drupal_get_js($scope = 'header', $javascript = NULL, $skip_alter = FALS
  *
  * Example:
  * @code
- * function module1_page_build(&$page) {
+ * function module1_page_attachments(&$page) {
  *   $page['#attached']['js'][] = array(
  *     'type' => 'setting',
  *     'data' => array('foo' => array('a', 'b', 'c')),
  *   );
  * }
- * function module2_page_build(&$page) {
+ * function module2_page_attachments(&$page) {
  *   $page['#attached']['js'][] = array(
  *     'type' => 'setting',
  *     'data' => array('foo' => array('d')),
@@ -2452,7 +2450,10 @@ function drupal_pre_render_links($element) {
  * @return array
  *   The processed render array for the page.
  *
- * @see hook_page_alter()
+ * @see hook_page_attachments()
+ * @see hook_page_attachments_alter()
+ * @see hook_page_top()
+ * @see hook_page_bottom()
  * @see element_info()
  */
 function drupal_prepare_page($page) {
@@ -2508,6 +2509,15 @@ function drupal_prepare_page($page) {
   // @todo Clean this up as part of https://www.drupal.org/node/2352155.
   if (\Drupal::moduleHandler()->moduleExists('block')) {
     _block_page_build($page);
+    // Find all non-empty page regions, and add a theme wrapper function that
+    // allows them to be consistently themed.
+    $regions = system_region_list(\Drupal::theme()->getActiveTheme()->getName());
+    foreach (array_keys($regions) as $region) {
+      if (!empty($page[$region])) {
+        $page[$region]['#theme_wrappers'][] = 'region';
+        $page[$region]['#region'] = $region;
+      }
+    }
   }
 
   // If no module has taken care of the main content, add it to the page now.
diff --git a/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php b/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php
index d4f4970..7139ec4 100644
--- a/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php
+++ b/core/lib/Drupal/Core/Page/DefaultHtmlFragmentRenderer.php
@@ -72,8 +72,12 @@ public function render(HtmlFragmentInterface $fragment, $status_code = 200) {
       // Persist cache tags associated with this page. Also associate the
       // "rendered" cache tag. This allows us to invalidate the entire render
       // cache, regardless of the cache bin.
-      $cache_tags = $page_array['#cache']['tags'];
-      $cache_tags[] = 'rendered';
+      $cache_tags = Cache::mergeTags(
+        isset($page_array['page_top']) ? $page_array['page_top']['#cache']['tags'] : [],
+        $page_array['#cache']['tags'],
+        isset($page_array['page_bottom']) ? $page_array['page_bottom']['#cache']['tags'] : [],
+        ['rendered']
+      );
       // Only keep unique cache tags. We need to prevent duplicates here already
       // rather than only in the cache layer, because they are also used by
       // reverse proxies (like Varnish), not only by Drupal's page cache.
diff --git a/core/modules/block/block.install b/core/modules/block/block.install
deleted file mode 100644
index a53a120..0000000
--- a/core/modules/block/block.install
+++ /dev/null
@@ -1,19 +0,0 @@
-<?php
-
-/**
- * @file
- * Install, update and uninstall functions for the block module.
- */
-
-use Drupal\Core\Language\Language;
-
-/**
- * Implements hook_install().
- */
-function block_install() {
-
-  // Block should go first so that other modules can alter its output
-  // during hook_page_alter(). Almost everything on the page is a block,
-  // so before block module runs, there will not be much to alter.
-  module_set_weight('block', -5);
-}
diff --git a/core/modules/content_translation/content_translation.module b/core/modules/content_translation/content_translation.module
index 4d3c794..b2f7698 100644
--- a/core/modules/content_translation/content_translation.module
+++ b/core/modules/content_translation/content_translation.module
@@ -722,9 +722,9 @@ function content_translation_preprocess_language_content_settings_table(&$variab
 }
 
 /**
- * Implements hook_page_alter().
+ * Implements hook_page_attachments().
  */
-function content_translation_page_alter(&$page) {
+function content_translation_page_attachments(&$page) {
   $route_match = \Drupal::routeMatch();
 
   // If the current route has no parameters, return.
diff --git a/core/modules/system/src/Tests/System/MainContentFallbackTest.php b/core/modules/system/src/Tests/System/MainContentFallbackTest.php
index 55fb6a9..288e681 100644
--- a/core/modules/system/src/Tests/System/MainContentFallbackTest.php
+++ b/core/modules/system/src/Tests/System/MainContentFallbackTest.php
@@ -61,19 +61,13 @@ function testMainContentFallback() {
     // Fallback should not trigger when another module is handling content.
     $this->drupalGet('system-test/main-content-handling');
     $this->assertRaw('id="system-test-content"', 'Content handled by another module');
-    $this->assertText(t('Content to test main content fallback'), 'Main content still displayed.');
+    $this->assertNoText(t('Content to test main content fallback'), 'Main content not displayed.');
 
     // Fallback should trigger when another module
     // indicates that it is not handling the content.
     $this->drupalGet('system-test/main-content-fallback');
     $this->assertText(t('Content to test main content fallback'), 'Main content fallback properly triggers.');
 
-    // Fallback should not trigger when another module is handling content.
-    // Note that this test ensures that no duplicate
-    // content gets created by the fallback.
-    $this->drupalGet('system-test/main-content-duplication');
-    $this->assertNoText(t('Content to test main content fallback'), 'Main content not duplicated.');
-
     // Request a user* page and see if it is displayed.
     $this->drupalLogin($this->web_user);
     $this->drupalGet('user/' . $this->web_user->id() . '/edit');
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index ee21a5e..41ad2c0 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -1268,21 +1268,6 @@ function system_entity_type_build(array &$entity_types) {
 }
 
 /**
- * Implements hook_page_alter().
- */
-function system_page_alter(&$page) {
-  // Find all non-empty page regions, and add a theme wrapper function that
-  // allows them to be consistently themed.
-  $regions = system_region_list(\Drupal::theme()->getActiveTheme()->getName());
-  foreach (array_keys($regions) as $region) {
-    if (!empty($page[$region])) {
-      $page[$region]['#theme_wrappers'][] = 'region';
-      $page[$region]['#region'] = $region;
-    }
-  }
-}
-
-/**
  * Implements hook_block_view_BASE_BLOCK_ID_alter().
  */
 function system_block_view_system_main_block_alter(array &$build, BlockPluginInterface $block) {
diff --git a/core/modules/system/tests/modules/system_test/system_test.module b/core/modules/system/tests/modules/system_test/system_test.module
index 40da5d2..78c1504 100644
--- a/core/modules/system/tests/modules/system_test/system_test.module
+++ b/core/modules/system/tests/modules/system_test/system_test.module
@@ -96,22 +96,21 @@ function system_test_lock_exit() {
 }
 
 /**
- * Implements hook_page_build().
+ * Implements hook_page_attachments().
  */
-function system_test_page_build(&$page) {
+function system_test_page_attachments(array &$page) {
   $menu_item['path'] = current_path();
   $main_content_display = &drupal_static('system_main_content_added', FALSE);
 
-  if ($menu_item['path'] == 'system-test/main-content-handling') {
-    $page['footer'] = drupal_set_page_content();
-    $page['footer']['main']['#markup'] = '<div id="system-test-content">' . $page['footer']['main']['#markup'] . '</div>';
-  }
-  elseif ($menu_item['path'] == 'system-test/main-content-fallback') {
+  if ($menu_item['path'] == 'system-test/main-content-fallback') {
+    // Get the main content, to e.g. dynamically attach an asset.
     drupal_set_page_content();
+    // Indicate we don't want to override the main content.
     $main_content_display = FALSE;
   }
-  elseif ($menu_item['path'] == 'system-test/main-content-duplication') {
-    drupal_set_page_content();
+  elseif ($menu_item['path'] == 'system-test/main-content-handling') {
+    // Set the main content.
+    drupal_set_page_content('<div id="system-test-content">Overridden!</div>');
   }
   // Used by FrontPageTestCase to get the results of drupal_is_front_page().
   $frontpage = \Drupal::state()->get('system_test.front_page_output') ?: 0;
diff --git a/core/modules/system/tests/modules/system_test/system_test.routing.yml b/core/modules/system/tests/modules/system_test/system_test.routing.yml
index 32d235b..9d257fb 100644
--- a/core/modules/system/tests/modules/system_test/system_test.routing.yml
+++ b/core/modules/system/tests/modules/system_test/system_test.routing.yml
@@ -29,14 +29,6 @@ system_test.main_content_fallback:
   requirements:
     _access: 'TRUE'
 
-system_test.main_content_duplication:
-  path: '/system-test/main-content-duplication'
-  defaults:
-    _title: 'Test main content duplication'
-    _content: '\Drupal\system_test\Controller\SystemTestController::mainContentFallback'
-  requirements:
-    _access: 'TRUE'
-
 system_test.lock_acquire:
   path: '/system-test/lock-acquire'
   defaults:
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index 8616f02..271b01d 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -74,10 +74,10 @@ function theme_test_preprocess_html(&$variables) {
 }
 
 /**
- * Implements hook_page_alter().
+ * Implements hook_page_bottom().
  */
-function theme_test_page_alter(&$page) {
-  $page['page_bottom']['theme_test_page_bottom'] = array('#markup' => 'theme test page bottom markup');
+function theme_test_page_bottom(array &$page_bottom) {
+  $page_bottom['theme_test_page_bottom'] = array('#markup' => 'theme test page bottom markup');
 }
 
 /**
diff --git a/core/modules/system/theme.api.php b/core/modules/system/theme.api.php
index 26c998a..d0d9749 100644
--- a/core/modules/system/theme.api.php
+++ b/core/modules/system/theme.api.php
@@ -161,14 +161,24 @@
  *
  * @section Assets
  *
- * We can distinguish between two types of assets:
- * 1. global assets (loaded on all pages where the theme is in use): these are
- *    defined in the theme's *.info.yml file.
- * 2. template-specific assets (loaded on all pages where a specific template is
+ * We can distinguish between threetypes of assets:
+ * 1. unconditional page-level assets (loaded on all pages where the theme is in
+ *    use): these are defined in the theme's *.info.yml file.
+ * 2. conditional page-level assets (loaded on all pages where the theme is in
+ *    use and a certain condition is met): these are attached in
+ *    hook_page_attachments_alter(), e.g.:
+ *    @code
+ *    function THEME_page_attachments_alter(array &$page) {
+ *      if ($some_condition) {
+ *        $page['#attached']['library'][] = 'mytheme/something';
+ *      }
+ *    }
+ *    @endcode
+ * 3. template-specific assets (loaded on all pages where a specific template is
  *    in use): these can be added by in preprocessing functions, using @code
  *    $variables['#attached'] @endcode, e.g.:
  *    @code
- *    function seven_preprocess_menu_local_action(array &$variables) {
+ *    function THEME_preprocess_menu_local_action(array &$variables) {
  *      // We require Modernizr's touch test for button styling.
  *      $variables['#attached']['library'][] = 'core/modernizr';
  *    }
diff --git a/core/modules/views/views.module b/core/modules/views/views.module
index a41e73e..5d0c6d6 100644
--- a/core/modules/views/views.module
+++ b/core/modules/views/views.module
@@ -285,15 +285,26 @@ function views_theme_suggestions_comment_alter(array &$suggestions, array $varia
 }
 
 /**
- * Implements hook_page_alter().
+ * Implements hook_element_info_alter().
+ *
+ * @see views_page_display_pre_render()
+ * @see views_preprocess_page()
+ */
+function views_element_info_alter(&$types) {
+  $types['page']['#pre_render'][] = 'views_page_display_pre_render';
+}
+
+/**
+ * #pre_render callback to set contextual links for views using a Page display.
  */
-function views_page_alter(&$page) {
+function views_page_display_pre_render(array $element) {
   // If the main content of this page contains a view, attach its contextual
   // links to the overall page array. This allows them to be rendered directly
   // next to the page title.
   if ($view = views_get_page_view()) {
-    views_add_contextual_links($page, 'page', $view, $view->current_display);
+    views_add_contextual_links($element, 'page', $view, $view->current_display);
   }
+  return $element;
 }
 
 /**
@@ -306,7 +317,8 @@ function views_preprocess_page(&$variables) {
   }
 
   // If the page contains a view as its main content, contextual links may have
-  // been attached to the page as a whole; for example, by views_page_alter().
+  // been attached to the page as a whole; for example, by
+  // views_page_display_pre_render().
   // This allows them to be associated with the page and rendered by default
   // next to the page title (which we want). However, it also causes the
   // Contextual Links module to treat the wrapper for the entire page (i.e.,