diff --git a/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php b/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
index 64a48c7..53b914b 100644
--- a/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
+++ b/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php
@@ -213,6 +213,10 @@ public function renderResponse(array $main_content, Request $request, RouteMatch
 
         ksort($placeholders);
 
+        // Hack: We now want all remaining placeholders to be processed directly.
+        global $process_remaining_placeholders;
+        $process_remaining_placeholders = TRUE;
+
         foreach ($placeholders as $placeholder => $placeholder_elements) {
           // Check if the placeholder is present at all.
           if (strpos($markup, $placeholder) === FALSE) {
diff --git a/core/lib/Drupal/Core/Render/Renderer.php b/core/lib/Drupal/Core/Render/Renderer.php
index acfb17f..a22fc6f 100644
--- a/core/lib/Drupal/Core/Render/Renderer.php
+++ b/core/lib/Drupal/Core/Render/Renderer.php
@@ -546,7 +546,7 @@ protected function processPostRenderCache(array &$elements) {
    *   The structured array describing the data being rendered.
    */
   protected function processPlaceholders(array &$elements) {
-    if (!isset($elements['#cache']['placeholders'])) {
+    if (empty($elements['#cache']['placeholders'])) {
       return;
     }
     foreach ($elements['#cache']['placeholders'] as $placeholder => $placeholder_elements) {
@@ -558,12 +558,16 @@ protected function processPlaceholders(array &$elements) {
         $elements['#markup'] = str_replace('<div id="' . $placeholder . '"></div>', $markup, $elements['#markup']);
         unset($elements['#cache']['placeholders'][$placeholder]);
       }
-/*
+
+      // @todo Render strategies will need to take care of this.
+      global $process_remaining_placeholders;
+
+      if (isset($process_remaining_placeholders)) {
         $placeholder_elements['#cache_no_placeholder'] = TRUE;
         $markup = $this->doRender($placeholder_elements);
-        $elements['#markup'] = str_replace($placeholder, $markup, $elements['#markup']);
+        $elements['#markup'] = str_replace('<div id="' . $placeholder . '"></div>', $markup, $elements['#markup']);
         unset($elements['#cache']['placeholders'][$placeholder]);
-*/
+      }
     }
   }
 
@@ -606,6 +610,15 @@ protected function cacheGet(array $elements, $is_recursive = FALSE) {
       }
     }
 
+    // Ensure that all placeholders can be 'satisfied for this request.
+    if (!empty($cached_element['#cache']['placeholders'])) {
+      if (!$this->checkPlaceholders($cached_element['#cache']['placeholders'])) {
+        // If placeholders are not cached AND don't use #pre_render_cache there
+        // is nothing we can do, treat the element as cache-miss.
+        $cached_element = FALSE;
+      }
+    }
+
     // Placeholder support: A bubbled up max-age=0 or high-frequency cache context
     // could have added a #cache_placeholder tag to the $cache_redirect entry.
     // Or the original $elements could have a max-age=0 or a #cache_placeholder tag.
@@ -640,6 +653,50 @@ protected function cacheGet(array $elements, $is_recursive = FALSE) {
   }
 
   /**
+   * Checks that all placeholders can be satisfied.
+   *
+   * A placeholder can be satisfied if it is:
+   *   - cached already for this request
+   *   - possible to re-create via #pre_render_cache
+   *
+   * @param array $placeholders
+   *  An array of placeholders keyed by placeholder id with a
+   *  render cache array as content.
+   *
+   * @return bool
+   *   TRUE if all placeholders can be satisfied, FALSE otherwise.
+   */
+  protected function checkPlaceholders(array $placeholders) {
+    // @todo: Use $this->getCacheMultiple() instead.
+    $cached_placeholders = [];
+    foreach ($placeholders as $placeholder => $elements) {
+      // If we can satisfy the information internally already, do so.
+      // This can happen if an earlier checkPlaceholders() failed
+      // (e.g. full page), but a lower level uses the same placeholder.
+      if (isset($this->placeholderData[$placeholder])) {
+        $cached_placeholders[$placeholder] = $this->placeholderData[$placeholder];
+        continue;
+      }
+      $cached_placeholders[$placeholder] = $this->cacheGet($elements);
+    }
+
+    $can_recreate_placeholders = TRUE;
+    foreach ($placeholders as $placeholder => $elements) {
+      if (!$cached_placeholders[$placeholder]) {
+        if (!isset($elements['#pre_render_cache'])) {
+          // This will make this fail, but still store the other placeholders.
+          $can_recreate_placeholders = FALSE;
+        }
+        continue;
+      }
+      $this->placeholderData[$placeholder] = $cached_placeholders[$placeholder];
+    }
+
+    // A cache miss and no #pre_render_cache is fatal.
+    return $can_recreate_placeholders;
+  }
+
+  /**
    * Create a render cache array placeholder.
    *
    * @param array $elements
diff --git a/core/modules/block/src/BlockViewBuilder.php b/core/modules/block/src/BlockViewBuilder.php
index c892150..6b3ad60 100644
--- a/core/modules/block/src/BlockViewBuilder.php
+++ b/core/modules/block/src/BlockViewBuilder.php
@@ -116,6 +116,11 @@ public static function preRenderBlock($build, $context) {
 
     $build['#configuration']['label'] = SafeMarkup::checkPlain($configuration['label']);
 
+    // Give the plugin a chance to alter the configuration first.
+    if (method_exists($plugin, 'alterBuild')) {
+      $plugin->alterBuild($build);
+    }
+
     // Don't run in ::buildBlock() to ensure cache keys can be altered. If an
     // alter hook wants to modify the block contents, it can append another
     // #pre_render hook.
diff --git a/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php b/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php
index a86fccc..77c354a 100644
--- a/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php
@@ -187,10 +187,30 @@ public function build() {
     usleep(2000*1000);
 
     // Bubble up that this is uncacheable.
+/*
     $build['site_slogan']['#cache']['max-age'] = 0;
     $build['site_slogan']['#cache']['contexts'] = ['user'];
     $build['site_slogan']['#debug'] = TRUE;
+*/
+    $build['site_slogan']['#cache']['contexts'] = ['user'];
+
+//    $build['site_slogan']['dynamic'] = [ '#markup' => '<div>It is: ' . time(NULL) . '</div>' ];
+    $build['site_slogan']['dynamic'] = [ '#pre_render_cache' => [get_class($this) . '::buildDynamic' => []] ];
+    $build['site_slogan']['dynamic']['#cache']['max-age'] = 0;
+
+    // This is a stage, where a placeholder can be created.
+    $build['site_slogan']['dynamic']['#cache']['keys'] = [ 'fabian', 'site-branding-dynamic' ];
+
+    return $build;
+  }
+
+  public function alterBuild(&$build) {
+    $build['#cache_placeholder'] = TRUE;
+  }
 
+  public static function buildDynamic($build, $context) {
+    $build['#markup'] = '<div>It is: ' . time(NULL) . '</div>';
+    usleep(500*1000);
     return $build;
   }