.../src/EventSubscriber/SmartCacheSubscriber.php   | 59 ++++++++++------------
 .../RequestPolicy/DenyNonHtmlRequests.php          |  3 +-
 2 files changed, 30 insertions(+), 32 deletions(-)

diff --git a/core/modules/smart_cache/src/EventSubscriber/SmartCacheSubscriber.php b/core/modules/smart_cache/src/EventSubscriber/SmartCacheSubscriber.php
index 056ea88..3678a34 100644
--- a/core/modules/smart_cache/src/EventSubscriber/SmartCacheSubscriber.php
+++ b/core/modules/smart_cache/src/EventSubscriber/SmartCacheSubscriber.php
@@ -179,6 +179,13 @@ public function onResponse(FilterResponseEvent $event) {
       return;
     }
 
+    // There's no work left to be done if this is an uncacheable HTML response.
+    if ($response->getCacheableMetadata()->getCacheMaxAge() === 0) {
+      // The HTML response is uncacheable, mark it as such.
+      $response->headers->set('X-Drupal-SmartCache', 'UNCACHEABLE');
+      return;
+    }
+
     // Don't cache the HTML response if SmartCache's request subscriber did not
     // fire, because that means it's impossible to have a SmartCache cache hit.
     // (This can happen when the master request is for example a 403 or 404, in
@@ -202,43 +209,33 @@ public function onResponse(FilterResponseEvent $event) {
       return;
     }
 
-    // Get the cacheability metadata.
+    // Calculate the cacheability of this HTML response's SmartCache cache item.
     $html_cacheability = CacheableMetadata::createFromObject($response->getCacheableMetadata())
       ->addCacheContexts(['route'])
       ->addCacheTags(['rendered']);
-
-    // SmartCache only caches cacheable HTML responses, i.e. with a max-age > 0.
-    if ($html_cacheability->getCacheMaxAge() !== 0) {
-      $this->normalizeCacheability($html_cacheability);
-
-      // Get the contexts by which the current route's response must be varied.
-      $stored_cache_contexts = $request->attributes->get(self::ATTRIBUTE_CACHE_CONTEXTS_FOR_ROUTE);
-
-      // If the set of cache contexts is different, store the union of the
-      // already stored cache contexts and the contexts for this request.
-      if ($html_cacheability->getCacheContexts() !== $stored_cache_contexts) {
-        if (is_array($stored_cache_contexts)) {
-          $html_cacheability->addCacheContexts($stored_cache_contexts);
-          $this->normalizeCacheability($html_cacheability);
-        }
-        $this->smartContextsCache->set($request->attributes->get(self::ATTRIBUTE_CACHE_CONTEXTS_FOR_ROUTE_CID), $html_cacheability->getCacheContexts());
+    $this->normalizeCacheability($html_cacheability);
+
+    // If the set of cache contexts of this HTML response is different from the
+    // already stored cache contexts, store the union of both.
+    $stored_cache_contexts = $request->attributes->get(self::ATTRIBUTE_CACHE_CONTEXTS_FOR_ROUTE);
+    if ($html_cacheability->getCacheContexts() !== $stored_cache_contexts) {
+      if (is_array($stored_cache_contexts)) {
+        $html_cacheability->addCacheContexts($stored_cache_contexts);
+        $this->normalizeCacheability($html_cacheability);
       }
+      $this->smartContextsCache->set($request->attributes->get(self::ATTRIBUTE_CACHE_CONTEXTS_FOR_ROUTE_CID), $html_cacheability->getCacheContexts());
+    }
 
-      // Finally, cache the HTML response by those contexts.
-      $context_cache_keys = $this->cacheContextsManager->convertTokensToKeys($html_cacheability->getCacheContexts());
-      $cid = implode(':', $context_cache_keys->getKeys());
-      $html_cacheability = $html_cacheability->merge($context_cache_keys);
-      $expire = ($html_cacheability->getCacheMaxAge() === Cache::PERMANENT) ? Cache::PERMANENT : (int) $request->server->get('REQUEST_TIME') + $html_cacheability->getCacheMaxAge();
-      $this->smartHtmlCache->set($cid, $response, $expire, $html_cacheability->getCacheTags());
+    // Finally, cache the HTML response by those contexts.
+    $context_cache_keys = $this->cacheContextsManager->convertTokensToKeys($html_cacheability->getCacheContexts());
+    $cid = implode(':', $context_cache_keys->getKeys());
+    $html_cacheability = $html_cacheability->merge($context_cache_keys);
+    $expire = ($html_cacheability->getCacheMaxAge() === Cache::PERMANENT) ? Cache::PERMANENT : (int) $request->server->get('REQUEST_TIME') + $html_cacheability->getCacheMaxAge();
+    $this->smartHtmlCache->set($cid, $response, $expire, $html_cacheability->getCacheTags());
 
-      // The HTML response was generated, mark the response as a cache miss. The
-      // next time, it will be a cache hit.
-      $response->headers->set('X-Drupal-SmartCache', 'MISS');
-    }
-    else {
-      // The HTML response is uncacheable, mark it as such.
-      $response->headers->set('X-Drupal-SmartCache', 'UNCACHEABLE');
-    }
+    // The HTML response was generated, mark the response as a cache miss. The
+    // next time, it will be a cache hit.
+    $response->headers->set('X-Drupal-SmartCache', 'MISS');
   }
 
   /**
diff --git a/core/modules/smart_cache/src/PageCache/RequestPolicy/DenyNonHtmlRequests.php b/core/modules/smart_cache/src/PageCache/RequestPolicy/DenyNonHtmlRequests.php
index dfc5445..7a9e7a4 100644
--- a/core/modules/smart_cache/src/PageCache/RequestPolicy/DenyNonHtmlRequests.php
+++ b/core/modules/smart_cache/src/PageCache/RequestPolicy/DenyNonHtmlRequests.php
@@ -15,7 +15,8 @@
  * Deny non-HTML requests.
  *
  * The policy denies caching if the request has a request format other than HTML
- * or when that is HTML, but additionally a wrapper format is specified.
+ * or when that is HTML, but additionally a wrapper format is specified. For
+ * example, 'drupal_ajax', 'drupal_modal' are wrapper formats.
  *
  * @see \Drupal\smart_cache\EventSubscriber\SmartCacheSubscriber
  */