core/core.services.yml                             |  2 +-
 .../Core/EventSubscriber/SmartCacheSubscriber.php  |  5 +++
 .../Render/MainContent/SmartCacheHtmlRenderer.php  | 39 ++++++++++++++++------
 3 files changed, 34 insertions(+), 12 deletions(-)

diff --git a/core/core.services.yml b/core/core.services.yml
index 09ab1c2..e00b79b 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -856,7 +856,7 @@ services:
       - { name: event_subscriber }
   main_content_renderer.html:
     class: Drupal\Core\Render\MainContent\SmartCacheHtmlRenderer
-    arguments: ['@title_resolver', '@plugin.manager.display_variant', '@event_dispatcher', '@module_handler', '@renderer', '@cache_contexts', '@current_route_match', '@cache.smart_cache_contexts', '@cache.smart_cache_html']
+    arguments: ['@title_resolver', '@plugin.manager.display_variant', '@event_dispatcher', '@module_handler', '@renderer', '@cache_contexts', '@current_route_match', '@cache.smart_cache_contexts', '@cache.smart_cache_html', '@request_stack']
     tags:
       - { name: render.main_content_renderer, format: html }
   main_content_renderer.ajax:
diff --git a/core/lib/Drupal/Core/EventSubscriber/SmartCacheSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/SmartCacheSubscriber.php
index 9fb0d66..0dc4fc0 100644
--- a/core/lib/Drupal/Core/EventSubscriber/SmartCacheSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/SmartCacheSubscriber.php
@@ -76,6 +76,11 @@ public function __construct(RouteMatchInterface $route_match, CacheContexts $cac
    *   The event to process.
    */
   public function onRouteMatch(GetResponseEvent $event) {
+    // SmartCache only supports master requests that are safe and ask for HTML.
+    if (!$event->isMasterRequest() || !$event->getRequest()->isMethodSafe() || $event->getRequest()->getRequestFormat() !== 'html') {
+      return;
+    }
+
     $this->routeMatch->getRouteName();
 
     // Smart cache caches per route.
diff --git a/core/lib/Drupal/Core/Render/MainContent/SmartCacheHtmlRenderer.php b/core/lib/Drupal/Core/Render/MainContent/SmartCacheHtmlRenderer.php
index bccd9f6..aab13c6 100644
--- a/core/lib/Drupal/Core/Render/MainContent/SmartCacheHtmlRenderer.php
+++ b/core/lib/Drupal/Core/Render/MainContent/SmartCacheHtmlRenderer.php
@@ -18,6 +18,7 @@
 use Drupal\Core\Routing\RouteMatchInterface;
 use Symfony\Component\DependencyInjection\ContainerAwareTrait;
 use Symfony\Component\EventDispatcher\EventDispatcherInterface;
+use Symfony\Component\HttpFoundation\RequestStack;
 
 /**
  * SmartCache main content renderer for HTML requests.
@@ -46,6 +47,13 @@ class SmartCacheHtmlRenderer extends HtmlRenderer {
   protected $smartHtmlCache;
 
   /**
+   * The request stack.
+   *
+   * @var \Symfony\Component\HttpFoundation\RequestStack
+   */
+  protected $requestStack;
+
+  /**
    * Constructs a new SmartCacheHtmlRenderer.
    *
    * @param \Drupal\Core\Controller\TitleResolverInterface $title_resolver
@@ -66,12 +74,15 @@ class SmartCacheHtmlRenderer extends HtmlRenderer {
    *   The Smart Cache contexts cache bin.
    * @param \Drupal\Core\Cache\CacheBackendInterface $html_cache
    *   The Smart Cache #type => html render array cache bin.
+   * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
+   *   The request stack.
    */
-  public function __construct(TitleResolverInterface $title_resolver, PluginManagerInterface $display_variant_manager, EventDispatcherInterface $event_dispatcher, ModuleHandlerInterface $module_handler, RendererInterface $renderer, CacheContexts $cache_contexts, RouteMatchInterface $route_match, CacheBackendInterface $contexts_cache, CacheBackendInterface $html_cache) {
+  public function __construct(TitleResolverInterface $title_resolver, PluginManagerInterface $display_variant_manager, EventDispatcherInterface $event_dispatcher, ModuleHandlerInterface $module_handler, RendererInterface $renderer, CacheContexts $cache_contexts, RouteMatchInterface $route_match, CacheBackendInterface $contexts_cache, CacheBackendInterface $html_cache, RequestStack $request_stack) {
     parent::__construct($title_resolver, $display_variant_manager, $event_dispatcher, $module_handler, $renderer, $cache_contexts);
     $this->routeMatch = $route_match;
     $this->smartContextsCache = $contexts_cache;
     $this->smartHtmlCache = $html_cache;
+    $this->requestStack = $request_stack;
   }
 
   /**
@@ -102,11 +113,13 @@ protected function finish(array $html) {
     // contexts, if any), and the total set of cache tags (to associate with the
     // smart_cache_html cache item).
     $cache_contexts = [];
+    $html_cache_max_age = Cache::PERMANENT;
     $html_cache_tags = [];
     foreach (Element::children($cacheable_html) as $child) {
       $cacheable_html[$child] = $this->renderer->getCacheableRenderArray($cacheable_html[$child]);
       $cache_contexts = Cache::mergeContexts($cache_contexts, $cacheable_html[$child]['#cache']['contexts']);
       $html_cache_tags = Cache::mergeTags($html_cache_tags, $cacheable_html[$child]['#cache']['tags']);
+      $html_cache_max_age = Cache::mergeMaxAges($html_cache_max_age, $cacheable_html[$child]['#cache']['max-age']);
     }
 
     // @todo Remove this. Work-around to support the deep-render-array-scanning-
@@ -117,18 +130,22 @@ protected function finish(array $html) {
       $cacheable_html['page'][$page_region] = ['#preprocess_functions_messing_with_cacheability' => TRUE];
     }
 
-    // If the set of cache contexts is different, store the union of the already
-    // stored cache contexts and the contexts for this request.
-    if ($cache_contexts !== $stored_cache_contexts) {
-      if (is_array($stored_cache_contexts)) {
-        $cache_contexts = Cache::mergeContexts($cache_contexts, $stored_cache_contexts);
+    // SmartCache only caches cacheable pages.
+    if ($html_cache_max_age !== 0) {
+      // If the set of cache contexts is different, store the union of the already
+      // stored cache contexts and the contexts for this request.
+      if ($cache_contexts !== $stored_cache_contexts) {
+        if (is_array($stored_cache_contexts)) {
+          $cache_contexts = Cache::mergeContexts($cache_contexts, $stored_cache_contexts);
+        }
+        $this->smartContextsCache->set($contexts_cid, $cache_contexts);
       }
-      $this->smartContextsCache->set($contexts_cid, $cache_contexts);
-    }
 
-    // Finally, cache the html render array according to those contexts.
-    $cid = 'smartcache:html_render_array:' . $cid_part_route_match . implode(':', $this->cacheContexts->convertTokensToKeys($cache_contexts));
-    $this->smartHtmlCache->set($cid, $cacheable_html, Cache::PERMANENT, $html_cache_tags);
+      // Finally, cache the html render array according to those contexts.
+      $cid = 'smartcache:html_render_array:' . $cid_part_route_match . implode(':', $this->cacheContexts->convertTokensToKeys($cache_contexts));
+      $expire = ($html_cache_max_age === Cache::PERMANENT) ? Cache::PERMANENT : (int) $this->requestStack->getMasterRequest()->server->get('REQUEST_TIME') + $html_cache_max_age;
+      $this->smartHtmlCache->set($cid, $cacheable_html, $expire, $html_cache_tags);
+    }
 
     return parent::finish($cacheable_html);
   }