diff --git a/core/lib/Drupal/Core/Render/HtmlResponseAttachmentsProcessor.php b/core/lib/Drupal/Core/Render/HtmlResponseAttachmentsProcessor.php
index df18779..0b001c7 100644
--- a/core/lib/Drupal/Core/Render/HtmlResponseAttachmentsProcessor.php
+++ b/core/lib/Drupal/Core/Render/HtmlResponseAttachmentsProcessor.php
@@ -161,7 +161,7 @@ public function processAttachments(AttachmentsInterface $response) {
       ['html_head', 'feed', 'html_head_link', 'http_header', 'library', 'html_response_attachment_placeholders', 'placeholders', 'drupalSettings']
     );
     if (!empty($unsupported_types)) {
-      throw new \LogicException(sprintf('You are not allowed to use %s in #attached', implode(', ', $unsupported_types)));
+      throw new \LogicException(sprintf('You are not allowed to use %s in #attached.', implode(', ', $unsupported_types)));
     }
 
     // Get the placeholders from attached and then remove them.
@@ -212,7 +212,7 @@ public function processAttachments(AttachmentsInterface $response) {
     // Now replace the attachment placeholders.
     $this->renderHtmlResponseAttachmentPlaceholders($response, $attachment_placeholders, $variables);
 
-    // Set the HTTP headers on the response if any bubbled.
+    // Set the HTTP headers and status code on the response if any bubbled.
     if (!empty($attached['http_header'])) {
       $this->setHeaders($response, $attached['http_header']);
     }
@@ -318,6 +318,9 @@ protected function processAssetLibraries(array $attached, array $placeholders) {
   /**
    * Renders HTML response attachment placeholders.
    *
+   * This is the last step where all of the attachments are placed into the
+   * response object's contents.
+   *
    * @param \Drupal\Core\Render\HtmlResponse $response
    *   The HTML response to update.
    * @param array $placeholders
@@ -343,7 +346,13 @@ protected function renderHtmlResponseAttachmentPlaceholders(HtmlResponse $respon
    * @param \Drupal\Core\Render\HtmlResponse $response
    *   The HTML response to update.
    * @param array $headers
-   *   The headers to set.
+   *   The headers to set, as an array. The items in this array should be as
+   *   follows:
+   *   - The header name.
+   *   - The header value.
+   *   - (optional) Whether to replace a current value with the new one, or add
+   *     it to the others. If the value is not replaced, it will be appended,
+   *     resulting in a header like this: 'Header: value1,value2'
    */
   protected function setHeaders(HtmlResponse $response, array $headers) {
     foreach ($headers as $values) {
@@ -392,14 +401,13 @@ protected function processHtmlHead(array $html_head) {
    * depending on options in the render array. Processing it can add to both the
    * html_head and http_header sections.
    *
-   * Each head link is specified by a two-element array:
+   * @param array $html_head_link
+   *   The 'html_head_link' value of a render array. Each head link is specified
+   *   by a two-element array:
    *   - An array specifying the attributes of the link.
    *   - A boolean specifying whether the link should also be a Link: HTTP
    *     header.
    *
-   * @param array $html_head_link
-   *   The 'html_head_link' value of a render array.
-   *
    * @return array
    *   An ['#attached'] section of a render array. This allows us to easily
    *   merge the results with other render arrays. The array could contain the
@@ -432,6 +440,10 @@ protected function processHtmlHeadLink(array $html_head_link) {
   }
 
   /**
+   * Transform a 'feed' attachment into an 'html_head_link' attachment.
+   *
+   * The RSS feed is a special case of 'html_head_link', so we just turn it into
+   * one.
    *
    * @param array $attached_feed
    *   The ['#attached']['feed'] portion of a render array.
@@ -446,7 +458,7 @@ protected function processFeed($attached_feed) {
       $feed_link = [
         'href' => $item[0],
         'rel' => 'alternate',
-        'title' => empty($attached_feed[1]) ? '' : $item[1],
+        'title' => empty($item[1]) ? '' : $item[1],
         'type' => 'application/rss+xml',
       ];
       $html_head_link[] = [$feed_link, FALSE];
diff --git a/core/modules/system/src/Tests/Render/HtmlResponseAttachmentsTest.php b/core/modules/system/src/Tests/Render/HtmlResponseAttachmentsTest.php
index d8bcddb..d95b3fb 100644
--- a/core/modules/system/src/Tests/Render/HtmlResponseAttachmentsTest.php
+++ b/core/modules/system/src/Tests/Render/HtmlResponseAttachmentsTest.php
@@ -36,37 +36,31 @@ public function testAttachments() {
     $this->assertResponse(418);
     $this->assertHeader('X-Drupal-Cache', 'HIT');
 
-    // Test ['#attached']['html_head'] with various replacement rules.
-    $this->drupalGet('/render_attached_test/head');
-    $this->assertHeader('X-Test-Teapot', 'Teapot Mode Active');
-    $this->assertHeader('X-Test-Teapot-Replace', 'Teapot replaced');
-    $this->assertHeader('X-Test-Teapot-No-Replace', 'This value is not replaced,This one is added');
+    // Test ['#attached']['http_header'] with various replacement rules.
+    $this->drupalGet('/render_attached_test/header');
+    $this->assertTeapotHeaders();
     $this->assertHeader('X-Drupal-Cache', 'MISS');
     // Repeat for the cache.
-    $this->drupalGet('/render_attached_test/head');
+    $this->drupalGet('/render_attached_test/header');
     $this->assertHeader('X-Drupal-Cache', 'HIT');
 
     // Test ['#attached']['feed'].
     $this->drupalGet('/render_attached_test/feed');
-    // Discover the DOM element for the feed link.
-    $link = $this->xpath('//head/link[@href="test://url"]');
-    $this->assertEqual(1, count($link), 'Link has URL.');
-    // Reconcile the other attributes.
-    $link_attributes = [
-      'href' => 'test://url',
-      'rel' => 'alternate',
-      'type' => 'application/rss+xml',
-      'title' => '',
-     ];
-    $link = reset($link);
-    foreach ($link->attributes() as $attribute => $value) {
-      $this->assertEqual($value, $link_attributes[$attribute]);
-    }
+    $this->assertHeader('X-Drupal-Cache', 'MISS');
+    $this->assertFeed();
     // Repeat for the cache.
     $this->drupalGet('/render_attached_test/feed');
     $this->assertHeader('X-Drupal-Cache', 'HIT');
 
-    // Now repeat the process using drupal_process_attached().
+    // Test ['#attached']['html_head'].
+    $this->drupalGet('/render_attached_test/head');
+    $this->assertHeader('X-Drupal-Cache', 'MISS');
+    $this->assertHead();
+    // Repeat for the cache.
+    $this->drupalGet('/render_attached_test/head');
+    $this->assertHeader('X-Drupal-Cache', 'HIT');
+
+    // Now repeat all of the preceeding using drupal_process_attached().
     // Test ['#attached']['http_header] = ['Status', $code].
     $this->drupalGet('/render_attached_test/teapot_dpa');
     $this->assertResponse(418);
@@ -76,29 +70,29 @@ public function testAttachments() {
     $this->assertResponse(418);
     $this->assertHeader('X-Drupal-Cache', 'HIT');
 
-    // Test ['#attached']['html_head'] with various replacement rules.
-    $this->drupalGet('/render_attached_test/head_dpa');
-    $this->assertHeader('X-Test-Teapot', 'Teapot Mode Active');
-    $this->assertHeader('X-Test-Teapot-Replace', 'Teapot replaced');
-    $this->assertHeader('X-Test-Teapot-No-Replace', 'This value is not replaced,This one is added');
+    // Test ['#attached']['http_header'] with various replacement rules.
+    $this->drupalGet('/render_attached_test/header_dpa');
     $this->assertHeader('X-Drupal-Cache', 'MISS');
+    $this->assertTeapotHeaders();
     // Repeat for the cache.
-    $this->drupalGet('/render_attached_test/head_dpa');
+    $this->drupalGet('/render_attached_test/header_dpa');
     $this->assertHeader('X-Drupal-Cache', 'HIT');
 
     // Test ['#attached']['feed'].
     $this->drupalGet('/render_attached_test/feed_dpa');
-    // Discover the DOM element for the feed link.
-    $link = $this->xpath('//head/link[@href="test://url"]');
-    $this->assertEqual(1, count($link), 'Link has URL.');
-    // Reconcile the other attributes.
-    $link = reset($link);
-    foreach ($link->attributes() as $attribute => $value) {
-      $this->assertEqual($value, $link_attributes[$attribute]);
-    }
+    $this->assertHeader('X-Drupal-Cache', 'MISS');
+    $this->assertFeed();
     // Repeat for the cache.
     $this->drupalGet('/render_attached_test/feed_dpa');
     $this->assertHeader('X-Drupal-Cache', 'HIT');
+
+    // Test ['#attached']['html_head'].
+    $this->drupalGet('/render_attached_test/head_dpa');
+    $this->assertHeader('X-Drupal-Cache', 'MISS');
+    $this->assertHead();
+    // Repeat for the cache.
+    $this->drupalGet('/render_attached_test/head_dpa');
+    $this->assertHeader('X-Drupal-Cache', 'HIT');
   }
 
   /**
@@ -112,15 +106,60 @@ public function testSmartCache() {
     $this->drupalGet('');
     // Make sure our block is visible.
     $this->assertText('Headers handled by drupal_process_attached().');
-    // The header added in the block build should be present.
-    $this->assertHeader('X-DPA-Cache-Test', 'HIT');
+    // Test that all our attached items are present.
+    $this->assertFeed();
+    $this->assertHead();
+    $this->assertResponse(418);
+    $this->assertTeapotHeaders();
 
     // Reload the page, to test caching.
     $this->drupalGet('');
     // Make sure our block is visible.
     $this->assertText('Headers handled by drupal_process_attached().');
     // The header should be present again.
-    $this->assertHeader('X-DPA-Cache-Test', 'HIT');
+    $this->assertHeader('X-Test-Teapot', 'Teapot Mode Active');
+  }
+
+  /**
+   * Helper function to make assertions about added HTTP headers.
+   */
+  protected function assertTeapotHeaders() {
+    $this->assertHeader('X-Test-Teapot', 'Teapot Mode Active');
+    $this->assertHeader('X-Test-Teapot-Replace', 'Teapot replaced');
+    $this->assertHeader('X-Test-Teapot-No-Replace', 'This value is not replaced,This one is added');
+  }
+
+  /**
+   * Helper function to make assertions about the presence of an RSS feed.
+   */
+  protected function assertFeed() {
+    // Discover the DOM element for the feed link.
+    $test_meta = $this->xpath('//head/link[@href="test://url"]');
+    $this->assertEqual(1, count($test_meta), 'Link has URL.');
+    // Reconcile the other attributes.
+    $test_meta_attributes = [
+      'href' => 'test://url',
+      'rel' => 'alternate',
+      'type' => 'application/rss+xml',
+      'title' => 'Your RSS feed.',
+    ];
+    $test_meta = reset($test_meta);
+    foreach ($test_meta->attributes() as $attribute => $value) {
+      $this->assertEqual($value, $test_meta_attributes[$attribute]);
+    }
+  }
+
+  /**
+   * Helper function to make assertions about HTML head elements.
+   */
+  protected function assertHead() {
+    // Discover the DOM element for the meta link.
+    $test_meta = $this->xpath('//head/meta[@test-attribute="testy"]');
+    $this->assertEqual(1, count($test_meta), 'There\'s only one test attribute.');
+    // Grab the only DOM element.
+    $test_meta = reset($test_meta);
+    $test_meta_attributes = $test_meta->attributes();
+    $this->assertEqual($test_meta_attributes['test-attribute'], 'testy');
   }
 
 }
diff --git a/core/modules/system/tests/modules/render_attached_test/render_attached_test.routing.yml b/core/modules/system/tests/modules/render_attached_test/render_attached_test.routing.yml
index 40f8229..d3d578b 100644
--- a/core/modules/system/tests/modules/render_attached_test/render_attached_test.routing.yml
+++ b/core/modules/system/tests/modules/render_attached_test/render_attached_test.routing.yml
@@ -5,6 +5,13 @@ render_attached.teapot:
   requirements:
     _access: 'TRUE'
 
+render_attached.header:
+  path: '/render_attached_test/header'
+  defaults:
+    _controller: '\Drupal\render_attached_test\Controller\TestController::header'
+  requirements:
+    _access: 'TRUE'
+
 render_attached.head:
   path: '/render_attached_test/head'
   defaults:
@@ -26,6 +33,13 @@ render_attached.teapot_dpa:
   requirements:
     _access: 'TRUE'
 
+render_attached.header_dpa:
+  path: '/render_attached_test/header_dpa'
+  defaults:
+    _controller: '\Drupal\render_attached_test\Controller\TestController::headerDpa'
+  requirements:
+    _access: 'TRUE'
+
 render_attached.head_dpa:
   path: '/render_attached_test/head_dpa'
   defaults:
diff --git a/core/modules/system/tests/modules/render_attached_test/src/Controller/TestController.php b/core/modules/system/tests/modules/render_attached_test/src/Controller/TestController.php
index 01898ef..3d158b3 100644
--- a/core/modules/system/tests/modules/render_attached_test/src/Controller/TestController.php
+++ b/core/modules/system/tests/modules/render_attached_test/src/Controller/TestController.php
@@ -28,9 +28,9 @@ public function teapotHeaderStatus() {
    * Test attached HTML head rendering.
    *
    * @return array
-   *   A render array using the 'html_head' directive.
+   *   A render array using the 'http_head' directive.
    */
-  public function head() {
+  public function header() {
     $render = [];
     $render['#attached']['http_header'][] = ['X-Test-Teapot-Replace', 'This value gets replaced'];
     $render['#attached']['http_header'][] = ['X-Test-Teapot-Replace', 'Teapot replaced', TRUE];
@@ -41,6 +41,28 @@ public function head() {
   }
 
   /**
+   * Test attached HTML head rendering.
+   *
+   * @return array
+   *   A render array using the 'html_head' directive.
+   */
+  public function head() {
+    $head = [
+      [
+        '#tag' => 'meta',
+        '#attributes' => [
+          'test-attribute' => 'testy',
+        ],
+      ],
+      'test_head_attribute',
+    ];
+
+    $render = [];
+    $render['#attached']['html_head'][] = $head;
+    return $render;
+  }
+
+  /**
    * Test attached feed rendering.
    *
    * @return array
@@ -59,9 +81,18 @@ public function feed() {
    *   A generic render array.
    */
   public function teapotHeaderStatusDpa() {
-    $render = [];
-    $render['#attached']['http_header'][] = ['Status', "418 I'm a teapot."];
-    \drupal_process_attached($render);
+    \drupal_process_attached($this->teapotHeaderStatus());
+    return ['#markup' => "I'm some markup here to fool the kernel into rendering this page."];
+  }
+
+  /**
+   * Test attached HTML head rendering as a side-effect.
+   *
+   * @return array
+   *   A render array using the 'http_header' directive.
+   */
+  public function headerDpa() {
+    \drupal_process_attached($this->header());
     return ['#markup' => "I'm some markup here to fool the kernel into rendering this page."];
   }
 
@@ -72,13 +103,7 @@ public function teapotHeaderStatusDpa() {
    *   A render array using the 'html_head' directive.
    */
   public function headDpa() {
-    $render = [];
-    $render['#attached']['http_header'][] = ['X-Test-Teapot-Replace', 'This value gets replaced'];
-    $render['#attached']['http_header'][] = ['X-Test-Teapot-Replace', 'Teapot replaced', TRUE];
-    $render['#attached']['http_header'][] = ['X-Test-Teapot-No-Replace', 'This value is not replaced'];
-    $render['#attached']['http_header'][] = ['X-Test-Teapot-No-Replace', 'This one is added', FALSE];
-    $render['#attached']['http_header'][] = ['X-Test-Teapot', 'Teapot Mode Active'];
-    \drupal_process_attached($render);
+    \drupal_process_attached($this->head());
     return ['#markup' => "I'm some markup here to fool the kernel into rendering this page."];
   }
 
@@ -89,9 +114,7 @@ public function headDpa() {
    *   A render array using the 'feed' directive.
    */
   public function feedDpa() {
-    $render = [];
-    $render['#attached']['feed'][] = ['test://url', 'Your RSS feed.'];
-    \drupal_process_attached($render);
+    \drupal_process_attached($this->feed());
     return ['#markup' => "I'm some markup here to fool the kernel into rendering this page."];
   }
 
diff --git a/core/modules/system/tests/modules/render_attached_test/src/Plugin/Block/DrupalProcessAttachedBlock.php b/core/modules/system/tests/modules/render_attached_test/src/Plugin/Block/DrupalProcessAttachedBlock.php
index 9ab5400..3e7b342 100644
--- a/core/modules/system/tests/modules/render_attached_test/src/Plugin/Block/DrupalProcessAttachedBlock.php
+++ b/core/modules/system/tests/modules/render_attached_test/src/Plugin/Block/DrupalProcessAttachedBlock.php
@@ -7,8 +7,10 @@
 
 namespace Drupal\render_attached_test\Plugin\Block;
 
+use Drupal\render_attached_test\Controller\TestController;
 use Drupal\Core\Block\BlockBase;
 use Drupal\Core\Cache\Cache;
+use Drupal\Core\Render\BubbleableMetadata;
 
 /**
  * A block we can use to test caching of #attached headers.
@@ -26,8 +28,16 @@ class DrupalProcessAttachedBlock extends BlockBase {
    * {@inheritdoc}
    */
   public function build() {
-    // Use drupal_process_attached() to attach an HTTP header.
-    \drupal_process_attached(['#attached' => ['http_header' => [['X-DPA-Cache-Test', 'HIT']]]]);
+    // Grab test attachment fixtures from
+    // Drupal\render_attached_test\Controller\TestController.
+    $controller = new TestController();
+    $attached = BubbleableMetadata::mergeAttachments($controller->feed(), $controller->head());
+    $attached = BubbleableMetadata::mergeAttachments($attached, $controller->header());
+    $attached = BubbleableMetadata::mergeAttachments($attached, $controller->teapotHeaderStatus());
+
+    // Use drupal_process_attached() to attach all the #attached stuff.
+    drupal_process_attached($attached);
+
     // Return some arbitrary markup so the block doesn't disappear.
     return ['#markup' => 'Headers handled by drupal_process_attached().'];
   }