diff --git a/core/modules/basic_auth/tests/src/Functional/BasicAuthTest.php b/core/modules/basic_auth/tests/src/Functional/BasicAuthTest.php
index 68ed384c86..6bc8cc3abf 100644
--- a/core/modules/basic_auth/tests/src/Functional/BasicAuthTest.php
+++ b/core/modules/basic_auth/tests/src/Functional/BasicAuthTest.php
@@ -51,7 +51,7 @@ public function testBasicAuth() {
     $this->assertText($account->getAccountName(), 'Account name is displayed.');
     $this->assertSession()->statusCodeEquals(200);
     $this->mink->resetSessions();
-    $this->assertNull($this->getSession()->getResponseHeader('X-Drupal-Cache'));
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
     // Check that Cache-Control is not set to public.
     $this->assertSession()->responseHeaderNotContains('Cache-Control', 'public');
 
@@ -85,7 +85,7 @@ public function testBasicAuth() {
     $this->drupalGet($url);
     $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'MISS');
     $this->basicAuthGet($url, $account->getAccountName(), $account->pass_raw);
-    $this->assertNull($this->getSession()->getResponseHeader('X-Drupal-Cache'));
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
     // Check that Cache-Control is not set to public.
     $this->assertSession()->responseHeaderNotContains('Cache-Control', 'public');
   }
diff --git a/core/modules/dynamic_page_cache/tests/src/Functional/DynamicPageCacheIntegrationTest.php b/core/modules/dynamic_page_cache/tests/src/Functional/DynamicPageCacheIntegrationTest.php
index 5ff2651a95..d539a3babf 100644
--- a/core/modules/dynamic_page_cache/tests/src/Functional/DynamicPageCacheIntegrationTest.php
+++ b/core/modules/dynamic_page_cache/tests/src/Functional/DynamicPageCacheIntegrationTest.php
@@ -56,22 +56,31 @@ public function testDynamicPageCache() {
     // Cache.
     $url = Url::fromUri('route:dynamic_page_cache_test.response');
     $this->drupalGet($url);
-    $this->assertNull($this->getSession()->getResponseHeader(DynamicPageCacheSubscriber::HEADER));
+    // Verify that response object is returned: Dynamic Page Cache is ignoring.
+    $this->assertSession()->responseHeaderDoesNotExist(DynamicPageCacheSubscriber::HEADER);
 
     // Controllers returning CacheableResponseInterface (cacheable response)
     // objects are handled by Dynamic Page Cache.
     $url = Url::fromUri('route:dynamic_page_cache_test.cacheable_response');
     $this->drupalGet($url);
+    // Verify that cacheable response object is returned: Dynamic Page Cache is active,
+    // Dynamic Page Cache MISS.
     $this->assertSession()->responseHeaderEquals(DynamicPageCacheSubscriber::HEADER, 'MISS');
     $this->drupalGet($url);
+    // Verify that cacheable response object is returned: Dynamic Page Cache is active,
+    // Dynamic Page Cache HIT.
     $this->assertSession()->responseHeaderEquals(DynamicPageCacheSubscriber::HEADER, 'HIT');
 
     // Controllers returning render arrays, rendered as HTML responses, are
     // handled by Dynamic Page Cache.
     $url = Url::fromUri('route:dynamic_page_cache_test.html');
     $this->drupalGet($url);
+    // Verify that 'render array is returned, rendered as HTML response:
+    // Dynamic Page Cache is active, Dynamic Page Cache MISS.
     $this->assertSession()->responseHeaderEquals(DynamicPageCacheSubscriber::HEADER, 'MISS');
     $this->drupalGet($url);
+    // Verify that 'render array is returned, rendered as HTML response:
+    // Dynamic Page Cache is active, Dynamic Page Cache HIT.
     $this->assertSession()->responseHeaderEquals(DynamicPageCacheSubscriber::HEADER, 'HIT');
 
     // The above is the simple case, where the render array returned by the
@@ -82,9 +91,13 @@ public function testDynamicPageCache() {
       $url = Url::fromUri('route:dynamic_page_cache_test.html.with_cache_contexts', ['query' => ['animal' => $animal]]);
       $this->drupalGet($url);
       $this->assertRaw($animal);
+      // Verify that 'render array is returned, rendered as HTML response:
+      // Dynamic Page Cache is active, Dynamic Page Cache MISS.
       $this->assertSession()->responseHeaderEquals(DynamicPageCacheSubscriber::HEADER, 'MISS');
       $this->drupalGet($url);
       $this->assertRaw($animal);
+      // Verify that 'render array is returned, rendered as HTML response:
+      // Dynamic Page Cache is active, Dynamic Page Cache HIT.
       $this->assertSession()->responseHeaderEquals(DynamicPageCacheSubscriber::HEADER, 'HIT');
 
       // Finally, let's also verify that the 'dynamic_page_cache_test.html'
@@ -93,6 +106,8 @@ public function testDynamicPageCache() {
       // Cache hits.
       $url = Url::fromUri('route:dynamic_page_cache_test.html', ['query' => ['animal' => 'piglet']]);
       $this->drupalGet($url);
+      // Verify that 'render array is returned, rendered as HTML response:
+      // Dynamic Page Cache is active, Dynamic Page Cache HIT.
       $this->assertSession()->responseHeaderEquals(DynamicPageCacheSubscriber::HEADER, 'HIT');
     }
 
@@ -100,31 +115,48 @@ public function testDynamicPageCache() {
     // response, are ignored by Dynamic Page Cache (but only because those
     // wrapper formats' responses do not implement CacheableResponseInterface).
     $this->drupalGet('dynamic-page-cache-test/html', ['query' => [MainContentViewSubscriber::WRAPPER_FORMAT => 'drupal_ajax']]);
-    $this->assertNull($this->getSession()->getResponseHeader(DynamicPageCacheSubscriber::HEADER));
+    // Verify that a Render array is returned, rendered as AJAX response:
+    // Dynamic Page Cache is ignoring.
+    $this->assertSession()->responseHeaderDoesNotExist(DynamicPageCacheSubscriber::HEADER);
     $this->drupalGet('dynamic-page-cache-test/html', ['query' => [MainContentViewSubscriber::WRAPPER_FORMAT => 'drupal_dialog']]);
-    $this->assertNull($this->getSession()->getResponseHeader(DynamicPageCacheSubscriber::HEADER));
+    // Verify that a Render array is returned, rendered as drupal_dialog response:
+    // Dynamic Page Cache is ignoring.
+    $this->assertSession()->responseHeaderDoesNotExist(DynamicPageCacheSubscriber::HEADER);
     $this->drupalGet('dynamic-page-cache-test/html', ['query' => [MainContentViewSubscriber::WRAPPER_FORMAT => 'drupal_modal']]);
-    $this->assertNull($this->getSession()->getResponseHeader(DynamicPageCacheSubscriber::HEADER));
+    // Verify that a Render array is returned, rendered as modal response:
+    // Dynamic Page Cache is ignoring.
+    $this->assertSession()->responseHeaderDoesNotExist(DynamicPageCacheSubscriber::HEADER);
 
     // Admin routes are ignored by Dynamic Page Cache.
     $this->drupalGet('dynamic-page-cache-test/html/admin');
-    $this->assertNull($this->getSession()->getResponseHeader(DynamicPageCacheSubscriber::HEADER));
+    // Verify that a response is returned, rendered as HTML response, admin
+    // route: Dynamic Page Cache is ignoring.
+    $this->assertSession()->responseHeaderDoesNotExist(DynamicPageCacheSubscriber::HEADER);
     $this->drupalGet('dynamic-page-cache-test/response/admin');
-    $this->assertNull($this->getSession()->getResponseHeader(DynamicPageCacheSubscriber::HEADER));
+    // Verify that a response is returned, plain response, admin route: Dynamic
+    // Page Cache is ignoring.
+    $this->assertSession()->responseHeaderDoesNotExist(DynamicPageCacheSubscriber::HEADER);
     $this->drupalGet('dynamic-page-cache-test/cacheable-response/admin');
-    $this->assertNull($this->getSession()->getResponseHeader(DynamicPageCacheSubscriber::HEADER));
+    // Verify that a response is returned, cacheable response, admin route:
+    // Dynamic Page Cache is ignoring.
+    $this->assertSession()->responseHeaderDoesNotExist(DynamicPageCacheSubscriber::HEADER);
 
     // Max-age = 0 responses are ignored by Dynamic Page Cache.
     $this->drupalGet('dynamic-page-cache-test/html/uncacheable/max-age');
+    // Verify that a render array is returned, rendered as HTML response,
+    // but uncacheable: Dynamic Page Cache is running, but not caching.
     $this->assertSession()->responseHeaderEquals(DynamicPageCacheSubscriber::HEADER, 'UNCACHEABLE');
 
     // 'user' cache context responses are ignored by Dynamic Page Cache.
     $this->drupalGet('dynamic-page-cache-test/html/uncacheable/contexts');
+    // Verify that a render array is returned, rendered as HTML response,
+    // but uncacheable: Dynamic Page Cache is running, but not caching.
     $this->assertSession()->responseHeaderEquals(DynamicPageCacheSubscriber::HEADER, 'UNCACHEABLE');
 
     // 'current-temperature' cache tag responses are ignored by Dynamic Page
     // Cache.
     $this->drupalGet('dynamic-page-cache-test/html/uncacheable/tags');
+    // Verify that by default, Drupal has no auto-placeholdering cache tags.
     $this->assertSession()->responseHeaderEquals(DynamicPageCacheSubscriber::HEADER, 'MISS');
   }
 
diff --git a/core/modules/file/tests/src/Functional/DownloadTest.php b/core/modules/file/tests/src/Functional/DownloadTest.php
index 8cd0734852..503e41c0e6 100644
--- a/core/modules/file/tests/src/Functional/DownloadTest.php
+++ b/core/modules/file/tests/src/Functional/DownloadTest.php
@@ -75,7 +75,9 @@ protected function doPrivateFileTransferTest() {
     file_test_reset();
     file_test_set_return('download', ['x-foo' => 'Bar']);
     $this->drupalGet($url);
+    // Verify that header is set by file_test module on private download.
     $this->assertSession()->responseHeaderEquals('x-foo', 'Bar');
+    // Verify that page cache is disabled on private file download.
     $this->assertSession()->responseHeaderDoesNotExist('x-drupal-cache');
     $this->assertSession()->statusCodeEquals(200);
     // Ensure hook_file_download is fired correctly.
diff --git a/core/modules/image/tests/src/Functional/ImageFieldDisplayTest.php b/core/modules/image/tests/src/Functional/ImageFieldDisplayTest.php
index 62b8a1c26b..147a79b847 100644
--- a/core/modules/image/tests/src/Functional/ImageFieldDisplayTest.php
+++ b/core/modules/image/tests/src/Functional/ImageFieldDisplayTest.php
@@ -141,6 +141,7 @@ public function _testImageFieldFormatters($scheme) {
     $this->assertSession()->responseHeaderContains('X-Drupal-Cache-Tags', $file->getCacheTags()[0]);
     // @todo Remove in https://www.drupal.org/node/2646744.
     $this->assertCacheContext('url.site');
+    // Verify that no image style cache tags are found.
     $this->assertSession()->responseHeaderNotContains('X-Drupal-Cache-Tags', 'image_style:');
     $this->assertRaw($default_output, 'Image linked to file formatter displaying correctly on full node view.');
     // Verify that the image can be downloaded.
@@ -171,7 +172,8 @@ public function _testImageFieldFormatters($scheme) {
       '#height' => 20,
     ];
     $this->drupalGet('node/' . $nid);
-    $this->assertCacheTag($file->getCacheTags()[0]);
+    $this->assertSession()->responseHeaderContains('X-Drupal-Cache-Tags', $file->getCacheTags()[0]);
+    // Verify that no image style cache tags are found.
     $this->assertSession()->responseHeaderNotContains('X-Drupal-Cache-Tags', 'image_style:');
     $elements = $this->xpath(
       '//a[@href=:path]/img[@src=:url and @alt=:alt and @width=:width and @height=:height]',
@@ -366,6 +368,7 @@ public function testImageFieldDefaultImage() {
     // Verify that no image is displayed on the page by checking for the class
     // that would be used on the image field.
     $this->assertSession()->responseNotMatches('<div class="(.*?)field--name-' . strtr($field_name, '_', '-') . '(.*?)">', 'No image displayed when no image is attached and no default image specified.');
+    // Verify that no image style cache tags are found.
     $this->assertSession()->responseHeaderNotContains('X-Drupal-Cache-Tags', 'image_style:');
 
     // Add a default image to the public image field.
@@ -395,7 +398,8 @@ public function testImageFieldDefaultImage() {
     ];
     $default_output = str_replace("\n", NULL, $renderer->renderRoot($image));
     $this->drupalGet('node/' . $node->id());
-    $this->assertCacheTag($file->getCacheTags()[0]);
+    $this->assertSession()->responseHeaderContains('X-Drupal-Cache-Tags', $file->getCacheTags()[0]);
+    // Verify that no image style cache tags are found.
     $this->assertSession()->responseHeaderNotContains('X-Drupal-Cache-Tags', 'image_style:');
     $this->assertRaw($default_output, 'Default image displayed when no user supplied image is present.');
 
@@ -419,7 +423,8 @@ public function testImageFieldDefaultImage() {
     ];
     $image_output = str_replace("\n", NULL, $renderer->renderRoot($image));
     $this->drupalGet('node/' . $nid);
-    $this->assertCacheTag($file->getCacheTags()[0]);
+    $this->assertSession()->responseHeaderContains('X-Drupal-Cache-Tags', $file->getCacheTags()[0]);
+    // Verify that no image style cache tags are found.
     $this->assertSession()->responseHeaderNotContains('X-Drupal-Cache-Tags', 'image_style:');
     $this->assertNoRaw($default_output, 'Default image is not displayed when user supplied image is present.');
     $this->assertRaw($image_output, 'User supplied image is displayed.');
@@ -469,7 +474,8 @@ public function testImageFieldDefaultImage() {
     ];
     $default_output = str_replace("\n", NULL, $renderer->renderRoot($image));
     $this->drupalGet('node/' . $node->id());
-    $this->assertCacheTag($file->getCacheTags()[0]);
+    $this->assertSession()->responseHeaderContains('X-Drupal-Cache-Tags', $file->getCacheTags()[0]);
+    // Verify that no image style cache tags are found.
     $this->assertSession()->responseHeaderNotContains('X-Drupal-Cache-Tags', 'image_style:');
     $this->assertRaw($default_output, 'Default private image displayed when no user supplied image is present.');
   }
diff --git a/core/modules/language/tests/src/Functional/LanguageBrowserDetectionAcceptLanguageTest.php b/core/modules/language/tests/src/Functional/LanguageBrowserDetectionAcceptLanguageTest.php
index db3870a4e2..869cc4ade3 100644
--- a/core/modules/language/tests/src/Functional/LanguageBrowserDetectionAcceptLanguageTest.php
+++ b/core/modules/language/tests/src/Functional/LanguageBrowserDetectionAcceptLanguageTest.php
@@ -84,32 +84,32 @@ public function testAcceptLanguageEmptyDefault() {
 
     $this->drupalGet('/system-test/echo/language test', [], ['Accept-Language' => 'en']);
     $this->assertSession()->responseHeaderEquals('Content-Language', 'en');
-    $this->assertNull($this->getSession()->getResponseHeader('X-Drupal-Cache'));
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
 
     // Check with UK browser.
     $this->drupalGet('/system-test/echo/language test', [], ['Accept-Language' => 'en-UK,en']);
     $this->assertSession()->responseHeaderEquals('Content-Language', 'en');
-    $this->assertNull($this->getSession()->getResponseHeader('X-Drupal-Cache'));
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
 
     // Check with french browser.
     $this->drupalGet('/system-test/echo/language test', [], ['Accept-Language' => 'fr-FR,fr']);
     $this->assertSession()->responseHeaderEquals('Content-Language', 'fr');
-    $this->assertNull($this->getSession()->getResponseHeader('X-Drupal-Cache'));
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
 
     // Check with browser without language settings - should return fallback language.
     $this->drupalGet('/system-test/echo/language test', [], ['Accept-Language' => NULL]);
     $this->assertSession()->responseHeaderEquals('Content-Language', 'en');
-    $this->assertNull($this->getSession()->getResponseHeader('X-Drupal-Cache'));
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
 
     // Check with french browser again.
     $this->drupalGet('/system-test/echo/language test', [], ['Accept-Language' => 'fr-FR,fr']);
     $this->assertSession()->responseHeaderEquals('Content-Language', 'fr');
-    $this->assertNull($this->getSession()->getResponseHeader('X-Drupal-Cache'));
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
 
     // Check with UK browser.
     $this->drupalGet('/system-test/echo/language test', [], ['Accept-Language' => 'en-UK,en']);
     $this->assertSession()->responseHeaderEquals('Content-Language', 'en');
-    $this->assertNull($this->getSession()->getResponseHeader('X-Drupal-Cache'));
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
 
     // Check if prefixed URLs are still cached.
     $this->drupalGet('/en/system-test/echo/language test', [], ['Accept-Language' => 'en']);
diff --git a/core/modules/page_cache/tests/src/Functional/PageCacheTest.php b/core/modules/page_cache/tests/src/Functional/PageCacheTest.php
index 0d1a91865d..1732faa80a 100644
--- a/core/modules/page_cache/tests/src/Functional/PageCacheTest.php
+++ b/core/modules/page_cache/tests/src/Functional/PageCacheTest.php
@@ -131,14 +131,18 @@ public function testQueryParameterFormatRequests() {
     $accept_header_cache_url_with_json = Url::fromRoute('system_test.page_cache_accept_header', ['_format' => 'json']);
 
     $this->drupalGet($accept_header_cache_url);
+    // Verify that HTML page was not yet cached.
     $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'MISS');
     $this->drupalGet($accept_header_cache_url);
+    // Verify that HTML page was cached.
     $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'HIT');
     $this->assertRaw('<p>oh hai this is html.</p>', 'The correct HTML response was returned.');
 
     $this->drupalGet($accept_header_cache_url_with_json);
+    // Verify that JSON response was not yet cached.
     $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'MISS');
     $this->drupalGet($accept_header_cache_url_with_json);
+    // Verify that JSON response was cached.
     $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'HIT');
     $this->assertRaw('{"content":"oh hai this is json"}', 'The correct Json response was returned.');
 
@@ -242,6 +246,7 @@ public function testConditionalRequests() {
     $this->drupalLogin($user);
     $this->drupalGet('', [], ['If-Modified-Since' => $last_modified, 'If-None-Match' => $etag]);
     $this->assertSession()->statusCodeEquals(200);
+    // Verify that absence of Page was not cached.
     $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
   }
 
@@ -282,7 +287,7 @@ public function testPageCache() {
     $user = $this->drupalCreateUser();
     $this->drupalLogin($user);
     $this->drupalGet('system-test/set-header', ['query' => ['name' => 'Foo', 'value' => 'bar']]);
-    $this->assertNull($this->getSession()->getResponseHeader('X-Drupal-Cache'));
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
     $this->assertSession()->responseHeaderNotContains('Vary', 'cookie');
     $this->assertSession()->responseHeaderEquals('Cache-Control', 'must-revalidate, no-cache, private');
     $this->assertSession()->responseHeaderEquals('Expires', 'Sun, 19 Nov 1978 05:00:00 GMT');
@@ -373,7 +378,7 @@ public function testPageCacheAnonymous403404() {
       $this->drupalGet($content_url);
       $this->assertSession()->statusCodeEquals($code);
       $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'MISS');
-      $this->assertCacheTag('4xx-response');
+      $this->assertSession()->responseHeaderContains('X-Drupal-Cache-Tags', '4xx-response');
       $this->drupalGet($content_url);
       $this->assertSession()->statusCodeEquals($code);
       $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'HIT');
@@ -511,12 +516,12 @@ public function testCacheableResponseResponses() {
 
     // GET a URL, which would be marked as a cache miss if it were cacheable.
     $this->drupalGet('/system-test/respond-public-response');
-    $this->assertNull($this->getSession()->getResponseHeader('X-Drupal-Cache'));
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
     $this->assertSession()->responseHeaderEquals('Cache-Control', 'max-age=60, public');
 
     // GET it again, verify it's still not cached.
     $this->drupalGet('/system-test/respond-public-response');
-    $this->assertNull($this->getSession()->getResponseHeader('X-Drupal-Cache'));
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
     $this->assertSession()->responseHeaderEquals('Cache-Control', 'max-age=60, public');
 
     // GET a URL, which should be marked as a cache miss.
diff --git a/core/modules/system/tests/src/Functional/Form/ResponseTest.php b/core/modules/system/tests/src/Functional/Form/ResponseTest.php
index df42e9237c..c316a7625e 100644
--- a/core/modules/system/tests/src/Functional/Form/ResponseTest.php
+++ b/core/modules/system/tests/src/Functional/Form/ResponseTest.php
@@ -36,7 +36,9 @@ public function testFormResponse() {
     $content = Json::decode($this->getSession()->getPage()->getContent());
     $this->assertSession()->statusCodeEquals(200);
     $this->assertIdentical($edit['content'], $content, 'Response content matches');
+    // Verify that response was handled by kernel response subscriber.
     $this->assertSession()->responseHeaderEquals('X-Form-Test-Response-Event', 'invoked');
+    // Verify that response was handled by kernel middleware.
     $this->assertSession()->responseHeaderEquals('X-Form-Test-Stack-Middleware', 'invoked');
 
     $edit = [
@@ -47,7 +49,9 @@ public function testFormResponse() {
     $content = Json::decode($this->getSession()->getPage()->getContent());
     $this->assertSession()->statusCodeEquals(418);
     $this->assertIdentical($edit['content'], $content, 'Response content matches');
+    // Verify that response was handled by kernel response subscriber.
     $this->assertSession()->responseHeaderEquals('X-Form-Test-Response-Event', 'invoked');
+    // Verify that response was handled by kernel middleware.
     $this->assertSession()->responseHeaderEquals('X-Form-Test-Stack-Middleware', 'invoked');
   }
 
diff --git a/core/modules/system/tests/src/Functional/Routing/RouterTest.php b/core/modules/system/tests/src/Functional/Routing/RouterTest.php
index 1819c006fb..97f85ea24d 100644
--- a/core/modules/system/tests/src/Functional/Routing/RouterTest.php
+++ b/core/modules/system/tests/src/Functional/Routing/RouterTest.php
@@ -46,7 +46,7 @@ public function testFinishResponseSubscriber() {
     $this->assertEquals($headers['Content-language'], ['en']);
     $this->assertEquals($headers['X-Content-Type-Options'], ['nosniff']);
     $this->assertEquals($headers['X-Frame-Options'], ['SAMEORIGIN']);
-    $this->assertNull($this->getSession()->getResponseHeader('Vary'));
+    $this->assertSession()->responseHeaderDoesNotExist('Vary');
 
     $this->drupalGet('router_test/test2');
     $this->assertRaw('test2', 'The correct string was returned because the route was successful.');
diff --git a/core/modules/system/tests/src/Functional/Session/SessionTest.php b/core/modules/system/tests/src/Functional/Session/SessionTest.php
index 3fb08af1aa..4c8ca3c9c3 100644
--- a/core/modules/system/tests/src/Functional/Session/SessionTest.php
+++ b/core/modules/system/tests/src/Functional/Session/SessionTest.php
@@ -51,6 +51,7 @@ public function testSessionSaveRegenerate() {
     // Start a new session by setting a message.
     $this->drupalGet('session-test/set-message');
     $this->assertSessionCookie(TRUE);
+    // Verify that the session cookie is set as HttpOnly.
     $this->assertSession()->responseHeaderMatches('Set-Cookie', '/HttpOnly/i');
 
     // Verify that the session is regenerated if a module calls exit
@@ -199,8 +200,10 @@ public function testEmptyAnonymousSession() {
     $this->drupalGet('');
     $this->assertSessionCookie(FALSE);
     $this->assertSessionEmpty(FALSE);
-    $this->assertNull($this->getSession()->getResponseHeader('X-Drupal-Cache'));
+    // Verify that caching was bypassed.
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
     $this->assertText(t('This is a dummy message.'), 'Message was displayed.');
+    // Verify that session cookie was deleted.
     $this->assertSession()->responseHeaderMatches('Set-Cookie', '/SESS\w+=deleted/');
 
     // Verify that session was destroyed.
@@ -210,7 +213,7 @@ public function testEmptyAnonymousSession() {
     // $this->assertSessionEmpty(TRUE);
     $this->assertNoText(t('This is a dummy message.'), 'Message was not cached.');
     $this->assertSession()->responseHeaderEquals('X-Drupal-Cache', 'HIT');
-    $this->assertNull($this->getSession()->getResponseHeader('Set-Cookie'));
+    $this->assertSession()->responseHeaderDoesNotExist('Set-Cookie');
 
     // Verify that no session is created if drupal_save_session(FALSE) is called.
     $this->drupalGet('session-test/set-message-but-do-not-save');
diff --git a/core/modules/system/tests/src/Functional/System/ErrorHandlerTest.php b/core/modules/system/tests/src/Functional/System/ErrorHandlerTest.php
index f87b058daf..6416a486a6 100644
--- a/core/modules/system/tests/src/Functional/System/ErrorHandlerTest.php
+++ b/core/modules/system/tests/src/Functional/System/ErrorHandlerTest.php
@@ -143,7 +143,7 @@ public function testExceptionHandler() {
       ->save();
 
     $this->drupalGet('error-test/trigger-exception');
-    $this->assertNull($this->getSession()->getResponseHeader('X-Drupal-Cache'));
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
     $this->assertSession()->responseHeaderNotContains('Cache-Control', 'public');
     $this->assertSession()->statusCodeEquals(500);
     $this->assertNoErrorMessage($error_exception);
diff --git a/core/modules/user/tests/src/Functional/UserPasswordResetTest.php b/core/modules/user/tests/src/Functional/UserPasswordResetTest.php
index bab17dcf2f..fe51266b8a 100644
--- a/core/modules/user/tests/src/Functional/UserPasswordResetTest.php
+++ b/core/modules/user/tests/src/Functional/UserPasswordResetTest.php
@@ -96,11 +96,11 @@ public function testUserPasswordReset() {
     // Ensure that the current url does not contain the hash and timestamp.
     $this->assertUrl(Url::fromRoute('user.reset.form', ['uid' => $this->account->id()]));
 
-    $this->assertNull($this->getSession()->getResponseHeader('X-Drupal-Cache'));
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
 
     // Ensure the password reset URL is not cached.
     $this->drupalGet($resetURL);
-    $this->assertNull($this->getSession()->getResponseHeader('X-Drupal-Cache'));
+    $this->assertSession()->responseHeaderDoesNotExist('X-Drupal-Cache');
 
     // Check the one-time login page.
     $this->assertText($this->account->getAccountName(), 'One-time login page contains the correct username.');
diff --git a/core/profiles/standard/tests/src/Functional/StandardTest.php b/core/profiles/standard/tests/src/Functional/StandardTest.php
index 2dfab83053..20b3c1d600 100644
--- a/core/profiles/standard/tests/src/Functional/StandardTest.php
+++ b/core/profiles/standard/tests/src/Functional/StandardTest.php
@@ -180,21 +180,26 @@ public function testStandard() {
     $this->drupalLogin($this->adminUser);
     $url = Url::fromRoute('contact.site_page');
     $this->drupalGet($url);
+    // Verify that site-wide contact page cannot be cached by Dynamic Page
+    // Cache.
     $this->assertSession()->responseHeaderEquals(DynamicPageCacheSubscriber::HEADER, 'UNCACHEABLE');
 
     $url = Url::fromRoute('<front>');
     $this->drupalGet($url);
     $this->drupalGet($url);
+    // Verify that frontpage is cached by Dynamic Page Cache.
     $this->assertSession()->responseHeaderEquals(DynamicPageCacheSubscriber::HEADER, 'HIT');
 
     $url = Url::fromRoute('entity.node.canonical', ['node' => 1]);
     $this->drupalGet($url);
     $this->drupalGet($url);
+    // Verify that full node page is cached by Dynamic Page Cache.
     $this->assertSession()->responseHeaderEquals(DynamicPageCacheSubscriber::HEADER, 'HIT');
 
     $url = Url::fromRoute('entity.user.canonical', ['user' => 1]);
     $this->drupalGet($url);
     $this->drupalGet($url);
+    // Verify that user profile page is cached by Dynamic Page Cache.
     $this->assertSession()->responseHeaderEquals(DynamicPageCacheSubscriber::HEADER, 'HIT');
 
     // Make sure the editorial workflow is installed after enabling the