.../page_cache/src/StackMiddleware/PageCache.php      | 12 +++++++++++-
 core/modules/page_cache/src/Tests/PageCacheTest.php   | 19 +++++++++++++++++++
 .../src/Controller/SystemTestController.php           |  9 +++++++++
 .../tests/modules/system_test/system_test.routing.yml |  7 +++++++
 4 files changed, 46 insertions(+), 1 deletion(-)

diff --git a/core/modules/page_cache/src/StackMiddleware/PageCache.php b/core/modules/page_cache/src/StackMiddleware/PageCache.php
index 85a7275..b83433e 100644
--- a/core/modules/page_cache/src/StackMiddleware/PageCache.php
+++ b/core/modules/page_cache/src/StackMiddleware/PageCache.php
@@ -225,7 +225,17 @@ protected function fetch(Request $request, $type = self::MASTER_REQUEST, $catch
     $date = $response->getExpires()->getTimestamp();
     $expire = ($date > time()) ? $date : Cache::PERMANENT;
 
-    $tags = ($response instanceof CacheableResponseInterface) ? $response->getCacheableMetadata()->getCacheTags() : [];
+    $tags = [];
+    if ($response instanceof CacheableResponseInterface) {
+      $tags = $response->getCacheableMetadata()->getCacheTags();
+    }
+    // The above (superior) mechanism to retrieve a response's cache tags was
+    // introduced during the RC phase. This exists to maintain backwards
+    // compatibility.
+    // @todo Remove in Drupal 9.0.0.
+    elseif ($response->headers->has('X-Drupal-Cache-Tags')) {
+      $tags = explode(' ', $response->headers->get('X-Drupal-Cache-Tags'));
+    }
     $this->set($request, $response, $expire, $tags);
 
     // Mark response as a cache miss.
diff --git a/core/modules/page_cache/src/Tests/PageCacheTest.php b/core/modules/page_cache/src/Tests/PageCacheTest.php
index 262ab4c..17a1f73 100644
--- a/core/modules/page_cache/src/Tests/PageCacheTest.php
+++ b/core/modules/page_cache/src/Tests/PageCacheTest.php
@@ -111,6 +111,25 @@ function testPageCacheTagsIndependentFromCacheabilityHeaders() {
   }
 
   /**
+   * Tests that we retain BC: responses with X-Drupal-Cache-Tags headers should
+   * have cache tag-based invalidation support in the Page Cache.
+   */
+  function testBackwardsCompatibilityResponseWithXDrupalCacheTagsHeader() {
+    $path = 'system-test/backwards_compatibility_response_with_x_drupal_cache_tags_header';
+    $this->drupalGet($path);
+    $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'MISS');
+    $this->drupalGet($path);
+    $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'HIT');
+    $this->assertCacheTag('bar');
+
+    // Verify that invalidating cache tags in the X-Drupal-Cache-Tags header
+    // causes the corresponding Page Cache item to be invalidated.
+    Cache::invalidateTags(['bar']);
+    $this->drupalGet($path);
+    $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'MISS');
+  }
+
+  /**
    * Tests support for different cache items with different request formats
    * specified via a query parameter.
    */
diff --git a/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php b/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
index abd9022..6a29835 100644
--- a/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
+++ b/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
@@ -237,6 +237,15 @@ public function system_test_cache_maxage_page() {
   }
 
   /**
+   * A response with an X-Drupal-Cache-Tags header. For backwards compatibility.
+   *
+   * @todo Remove in Drupal 9.0.0.
+   */
+  public function backwardsCompatibilityResponseWithXDrupalCacheTagsHeader() {
+    return new Response('', 200, ['X-Drupal-Cache-Tags' => 'foo bar']);
+  }
+
+  /**
    * Sets a cache tag on an element to help test #pre_render and cache tags.
    */
   public static function preRenderCacheTags($elements) {
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 94a1bef..ca95c97 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
@@ -73,6 +73,13 @@ system_test.cache_maxage_page:
   requirements:
     _access: 'TRUE'
 
+system_test.backwards_compatibility_response_with_x_drupal_cache_tags_header:
+  path: '/system-test/backwards_compatibility_response_with_x_drupal_cache_tags_header'
+  defaults:
+    _controller: '\Drupal\system_test\Controller\SystemTestController::backwardsCompatibilityResponseWithXDrupalCacheTagsHeader'
+  requirements:
+    _access: 'TRUE'
+
 system_test.authorize_init:
   path: '/system-test/authorize-init/{page_title}'
   defaults: