diff --git a/core/lib/Drupal/Core/Render/AttachmentsResponseProcessorInterface.php b/core/lib/Drupal/Core/Render/AttachmentsResponseProcessorInterface.php
index 8257763..c0f9c65 100644
--- a/core/lib/Drupal/Core/Render/AttachmentsResponseProcessorInterface.php
+++ b/core/lib/Drupal/Core/Render/AttachmentsResponseProcessorInterface.php
@@ -9,6 +9,8 @@
 /**
  * Defines an interface for processing attachments of responses that have them.
  *
+ * @see \Drupal\Core\Ajax\AjaxResponse
+ * @see \Drupal\Core\Ajax\AjaxResponseAttachmentsProcessor
  * @see \Drupal\Core\Render\HtmlResponse
  * @see \Drupal\Core\Render\HtmlResponseAttachmentsProcessor
  */
@@ -17,13 +19,38 @@
   /**
    * Processes the attachments of a response that has attachments.
    *
+   * Libraries, JavaScript settings, feeds, HTML <head> tags, HTML <head> links,
+   * HTTP headers, and the HTTP status code are attached to render arrays using
+   * the #attached property. The #attached property is an associative array,
+   * where the keys are the attachment types and the values are the attached
+   * data. For example:
+   *
+   * @code
+   * $build['#attached']['library'][] = [
+   *   'library' => ['core/jquery']
+   * ];
+   * $build['#attached']['http_header'][] = [
+   *   ['Content-Type', 'application/rss+xml; charset=utf-8'],
+   * ];
+   * @endcode
+   *
+   * The available keys are:
+   * - 'library' (asset libraries)
+   * - 'drupalSettings' (JavaScript settings)
+   * - 'feed' (RSS feeds)
+   * - 'html_head' (tags in HTML <head>)
+   * - 'html_head_link' (<link> tags in HTML <head>)
+   * - 'http_header' (HTTP headers and status code)
+   *
    * @param \Drupal\Core\Render\AttachmentsInterface $response
-   *   The response to process the attachments for.
+   *   The response to process.
    *
    * @return \Drupal\Core\Render\AttachmentsInterface
    *   The processed response.
    *
    * @throws \InvalidArgumentException
+   *   Thrown when the $response parameter is not a
+   *   Drupal\Core\Render\HtmlResponse.
    */
   public function processAttachments(AttachmentsInterface $response);
 
diff --git a/core/lib/Drupal/Core/Render/HtmlResponseAttachmentsProcessor.php b/core/lib/Drupal/Core/Render/HtmlResponseAttachmentsProcessor.php
index b5b3c27..f777a53 100644
--- a/core/lib/Drupal/Core/Render/HtmlResponseAttachmentsProcessor.php
+++ b/core/lib/Drupal/Core/Render/HtmlResponseAttachmentsProcessor.php
@@ -17,6 +17,15 @@
 /**
  * Processes attachments of HTML responses.
  *
+ * This class is used by the rendering service to process the #attached part of
+ * the render array, for HTML responses. It replaces drupal_process_attached()
+ * and drupal_get_html_head() from Drupal 7.
+ *
+ * To render attachments to HTML for testing without a controller, use the
+ * 'bare_html_page_renderer' service to generate a
+ * Drupal\Core\Render\HtmlResponse object. Then use its getContent(),
+ * getStatusCode(), and/or the headers property to access the result.
+ *
  * @see template_preprocess_html()
  * @see \Drupal\Core\Render\AttachmentsResponseProcessorInterface
  * @see \Drupal\Core\Render\BareHtmlPageRenderer
@@ -123,12 +132,12 @@ public function processAttachments(AttachmentsInterface $response) {
     // placeholder for it, we can safely avoid processing the render array if
     // it's not present.
     if (!empty($placeholders['head'])) {
-
-      // feed is a special casse of html_head_link. We create an html_head_link
-      // for the RSS feed.
+      // 'feed' is a special case of 'html_head_link'. We create an
+      // 'html_head_link' for the RSS feed.
       if (!empty($attached['feed'])) {
         // @todo: Determine if feed is supposed to be a single structure or an
-        // array of structures.
+        // array of structures, and then refactor this array manipulation into
+        // a separate method if needed.
         $feed = $attached['feed'];
 
         if (is_array($feed[0])) {
@@ -153,33 +162,37 @@ public function processAttachments(AttachmentsInterface $response) {
         }
       }
 
-      // html_head_link is a special case of html_head which can be present as
-      // a head element, but also as a Link: http header depending on settings
-      // in the render array. Processing it can add to both the html_head and
-      // http_header keys of #attached, so we must address it first.
+      // 'html_head_link' is a special case of 'html_head' which can be present
+      // as a head element, but also as a Link: HTTP header depending on
+      // settings in the render array. Processing it can add to both the
+      // 'html_head' and 'http_header' keys of '#attached', so we must address
+      // it first.
       if (!empty($attached['html_head_link'])) {
-        $link_attached = $this->processHtmlHeadLink($attached['html_head_link']);
-        // Merge $link_attached into $attached so that its html_head and
-        // http_header values are present for further processing. The merge
-        // requires a full render array, so we have to add #attached keys.
+        // Merge the processed 'html_head_link' into $attached so that its
+        // 'html_head' and 'http_header' values are present for further
+        // processing. The merge requires a full render array, so we have to add
+        // '#attached' keys.
         $attached = $this->renderer->mergeBubbleableMetadata(
           ['#attached' => $attached],
-          ['#attached' => $link_attached]
+          ['#attached' => $this->processHtmlHeadLink($attached['html_head_link'])]
         );
-        // Unwrap the #attached key.
+        // Unwrap the '#attached' key.
         $attached = $attached['#attached'];
       }
 
-      // Now we can process html_head.
+      // Now we can process 'html_head', which contains both 'feed' and
+      // 'html_head_link'.
       if (!empty($attached['html_head'])) {
-        $head = $this->processHtmlHead($attached['html_head']);
+        $html_head = $this->processHtmlHead($attached['html_head']);
         // Invoke hook_html_head_alter().
-        $this->moduleHandler->alter('html_head', $head);
-        $variables['head'] = $head;
+        $this->moduleHandler->alter('html_head', $html_head);
+        // Store the result in $variables so it can be inserted into the
+        // placeholder.
+        $variables['head'] = $html_head;
       }
     }
 
-    // Replace the placeholders in the response content with the real data.
+    // Replace the placeholders in the response content with our rendered HTML.
     $this->renderPlaceholders($response, $placeholders, $variables);
 
     // Set the HTTP headers on the response if any bubbled.
diff --git a/core/lib/Drupal/Core/Render/theme.api.php b/core/lib/Drupal/Core/Render/theme.api.php
index 689dce0..42fe60e 100644
--- a/core/lib/Drupal/Core/Render/theme.api.php
+++ b/core/lib/Drupal/Core/Render/theme.api.php
@@ -355,7 +355,10 @@
  * Libraries, JavaScript settings, feeds, HTML <head> tags and HTML <head> links
  * are attached to elements using the #attached property. The #attached property
  * is an associative array, where the keys are the attachment types and the
- * values are the attached data. For example:
+ * values are the attached data.
+ *
+ * The #attached property can also be used to specify HTTP headers and the
+ * response status code.
  *
  * The #attached property allows loading of asset libraries (which may contain
  * CSS assets, JavaScript assets, and JavaScript setting assets), JavaScript
@@ -369,7 +372,8 @@
  * $build['#attached']['feed'][] = ['aggregator/rss', $this->t('Feed title')];
  * @endcode
  *
- * See drupal_process_attached() for additional information.
+ * See \Drupal\Core\Render\AttachmentsResponseProcessorInterface for additional
+ * information.
  *
  * See \Drupal\Core\Asset\LibraryDiscoveryParser::parseLibraryInfo() for more
  * information on how to define libraries.