diff --git a/core/includes/common.inc b/core/includes/common.inc
index c0109b9..9ec0e86 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -3568,19 +3568,11 @@ function drupal_prepare_page($page) {
     drupal_set_page_content($page);
     $page = element_info('page');
   }
-  // Special front controllers (like install.php and update.php) are returning a
-  // proper #type 'page' render array (in order to set a custom #theme template),
-  // so just ensure that #type 'page' is complete.
-  elseif (isset($page['#type']) && $page['#type'] === 'page') {
-    $page += element_info('page');
-  }
 
   // Modules can add elements to $page as needed in hook_page_build().
-  if (!defined('MAINTENANCE_MODE')) {
-    foreach (\Drupal::moduleHandler()->getImplementations('page_build') as $module) {
-      $function = $module . '_page_build';
-      $function($page);
-    }
+  foreach (\Drupal::moduleHandler()->getImplementations('page_build') as $module) {
+    $function = $module . '_page_build';
+    $function($page);
   }
   // Modules alter the $page as needed. Blocks are populated into regions like
   // 'sidebar_first', 'footer', etc.
@@ -3591,11 +3583,11 @@ function drupal_prepare_page($page) {
   // cache, even though they should be. This happens because they're rendered
   // directly by the theme system.
   // @todo Remove this once https://drupal.org/node/1869476 lands.
-  if (!defined('MAINTENANCE_MODE') && theme_get_setting('features.main_menu') && count(menu_main_menu())) {
+  if (theme_get_setting('features.main_menu') && count(menu_main_menu())) {
     $main_links_source = _menu_get_links_source('main_links', 'main');
     $page['page_top']['#cache']['tags']['menu'][$main_links_source] = $main_links_source;
   }
-  if (!defined('MAINTENANCE_MODE') && theme_get_setting('features.secondary_menu') && count(menu_secondary_menu())) {
+  if (theme_get_setting('features.secondary_menu') && count(menu_secondary_menu())) {
     $secondary_links_source = _menu_get_links_source('secondary_links', 'account');
     $page['page_top']['#cache']['tags']['menu'][$secondary_links_source] = $secondary_links_source;
   }
@@ -3603,7 +3595,7 @@ function drupal_prepare_page($page) {
   // If no module has taken care of the main content, add it to the page now.
   // This allows the site to still be usable even if no modules that
   // control page regions (for example, the Block module) are enabled.
-  if (!$main_content_display && !isset($page['content']['system_main'])) {
+  if (!$main_content_display) {
     $page['content']['system_main'] = drupal_set_page_content();
   }
 
diff --git a/core/lib/Drupal/Core/Page/DefaultHtmlPageRenderer.php b/core/lib/Drupal/Core/Page/DefaultHtmlPageRenderer.php
index 4ce4ab2..4358ead 100644
--- a/core/lib/Drupal/Core/Page/DefaultHtmlPageRenderer.php
+++ b/core/lib/Drupal/Core/Page/DefaultHtmlPageRenderer.php
@@ -33,17 +33,31 @@ public function render(HtmlPage $page) {
    * Do NOT use this method in your code. This method will be removed as soon
    * as architecturally possible.
    *
+   * This is functionally very similar to DefaultHtmlFragmentRenderer::render()
+   * but with the following important differences:
+   *
+   * - drupal_prepare_page() and hook_page_build() cannot be invoked on the
+   *   maintenance and install pages, since possibly enabled page layout/block
+   *   modules would replace the main page content with configured region
+   *   content.
+   * - This function composes a complete page render array including a page
+   *   template theme suggestion (as opposed to the main page content only).
+   * - The render cache and cache tags is skipped.
+   *
    * @param array|string $main
    *   A render array or string containing the main page content.
    * @param string $title
    *   (optional) The page title.
    * @param string $theme
-   *   (optional) The theme hook to use for rendering the page. The given value
-   *   is appended with '_page' at this point, since ultimately this parameter
-   *   will be converted into a theme template suggestion; i.e., #theme becomes
-   *   'page--$theme'. Defaults to 'maintenance'.
+   *   (optional) The theme hook to use for rendering the page.  Defaults to
+   *   'maintenance'. The given value will be appended with '_page' to compose
+   *   the #theme property for #type 'page' currently; e.g., 'maintenance'
+   *   becomes 'maintenance_page'. Ultimately this parameter will be converted
+   *   into a page template theme suggestion; i.e., 'page__$theme'.
    * @param array $regions
-   *   (optional) Additional region content to add to the page.
+   *   (optional) Additional region content to add to the page. The given array
+   *   is added to the page render array, so this parameter may also be used to
+   *   pass e.g. the #show_messages property for #type 'page'.
    *
    * @return string
    *   The rendered HTML page.
@@ -51,27 +65,40 @@ public function render(HtmlPage $page) {
    * @internal
    */
   public static function renderPage($main, $title = '', $theme = 'maintenance', array $regions = array()) {
+    // Automatically convert the main page content into a render array.
     if (!is_array($main)) {
       $main = array('#markup' => $main);
     }
     $page = new HtmlPage('', array(), $title);
     $page_array = array(
       '#type' => 'page',
-      // @todo Change this into "page__$theme" and rename templates into
-      //   page--maintenance and page--install.
+      // @todo Change into theme suggestions "page__$theme".
       '#theme' => $theme . '_page',
       '#title' => $title,
       'content' => array(
         'system_main' => $main,
       ),
     );
+    // Append region content.
     $page_array += $regions;
+    // Add default properties.
     $page_array += element_info('page');
 
+    // hook_page_build() cannot be invoked on the maintenance and install pages,
+    // because the application is in an unknown or special state.
+    // In particular on the install page, invoking hook_page_build() directly
+    // after e.g. Block module has been installed would *replace* the installer
+    // output with the configured blocks of the installer theme (loaded from
+    // default configuration of the installation profile).
+
     // Allow modules and themes to alter the page render array.
+    // This allows e.g. themes to attach custom libraries.
     \Drupal::moduleHandler()->alter('page', $page_array);
 
+    // @todo Move preparePage() before alter() above, so $page_array['#page'] is
+    //   available in hook_page_alter(), so that HTML attributes can be altered.
     $page = \Drupal::service('html_fragment_renderer')->preparePage($page, $page_array);
+
     $page->setBodyTop(drupal_render($page_array['page_top']));
     $page->setBodyBottom(drupal_render($page_array['page_bottom']));
     $page->setContent(drupal_render($page_array));