.../Drupal/Core/EventSubscriber/FinishResponseSubscriber.php |  6 +++---
 core/modules/simpletest/src/WebTestBase.php                  |  3 +--
 sites/example.settings.local.php                             | 12 +++++++-----
 3 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php
index 3f22250..48b53bd 100644
--- a/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/FinishResponseSubscriber.php
@@ -116,9 +116,9 @@ public function onRespond(FilterResponseEvent $event) {
     $response->headers->set('X-Frame-Options', 'SAMEORIGIN', FALSE);
 
     // Expose the cache contexts and cache tags associated with this page in a
-    // X-Drupal-Cache-Contexts and X-Drupal-Cache-Tags header respectively.
-    // We just want to send the cacheability headers in case it is actually
-    // needed, so a reverse proxy is used, or when we want to debug its values.
+    // X-Drupal-Cache-Contexts and X-Drupal-Cache-Tags header respectively, when
+    // either a reverse proxy is being used (so the reverse proxy or CDN can be
+    // invalidated when appropriate) or when developing/debugging.
     if ((Settings::get('reverse_proxy', FALSE) || Settings::get('send_cacheability_headers', FALSE)) && $response instanceof CacheableResponseInterface) {
       $response_cacheability = $response->getCacheableMetadata();
       $response->headers->set('X-Drupal-Cache-Tags', implode(' ', $response_cacheability->getCacheTags()));
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index c053110..dad9dbf 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -743,8 +743,7 @@ protected function prepareSettings() {
       'required' => TRUE,
     ];
 
-    // Enable to send cacheability metadata headers for tests in order to check
-    // their values.
+    // Send cacheability headers so tests can check their values.
     $settings['settings']['send_cacheability_headers'] = (object) [
       'value' => TRUE,
       'required' => TRUE,
diff --git a/sites/example.settings.local.php b/sites/example.settings.local.php
index 8abdb00..0711fb9 100644
--- a/sites/example.settings.local.php
+++ b/sites/example.settings.local.php
@@ -59,6 +59,13 @@
 $settings['cache']['bins']['render'] = 'cache.backend.null';
 
 /**
+ * Send cacheablity headers for debugging purposes.
+ *
+ * By default, cacheability headers are only sent when behind a reverse proxy.
+ */
+$settings['send_cacheability_headers'] = TRUE;
+
+/**
  * Allow test modules and themes to be installed.
  *
  * Drupal ignores test modules and themes by default for performance reasons.
@@ -76,8 +83,3 @@
  * using these parameters in a request to rebuild.php.
  */
 $settings['rebuild_access'] = TRUE;
-
-/**
- * Send cacheablity metadata for debugging purposes.
- */
-$settings['send_cacheability_headers'] = TRUE;