reverted:
--- b/core/modules/field/tests/src/Kernel/Boolean/BooleanFormatterTest.php
+++ a/core/modules/field/tests/src/Kernel/Boolean/BooleanFormatterTest.php
@@ -138,7 +138,7 @@
 
       // Verify that all HTML is escaped and newlines are retained.
       $this->renderEntityFields($entity, $this->display);
+      $this->assertRaw($expected);
-      $this->assertSession()->responseContains($expected);
     }
   }
 
reverted:
--- b/core/modules/field/tests/src/Kernel/DisplayApiTest.php
+++ a/core/modules/field/tests/src/Kernel/DisplayApiTest.php
@@ -172,7 +172,7 @@
     $build = $items->view($display);
     $this->render($build);
     $setting = $display['settings']['test_formatter_setting_multiple'];
+    $this->assertRaw('visually-hidden', 'Label was visually hidden.');
-    $this->assertSession()->responseContains('visually-hidden', 'Label was visually hidden.');
     $this->assertText('field_test_entity_display_build_alter', 'Alter fired, display passed.');
     $this->assertText('entity language is en', 'Language is placed onto the context.');
     $array = [];
reverted:
--- b/core/modules/field/tests/src/Kernel/FieldAttachOtherTest.php
+++ a/core/modules/field/tests/src/Kernel/FieldAttachOtherTest.php
@@ -65,13 +65,13 @@
     // View all fields.
     $content = $display->build($entity);
     $this->render($content);
+    $this->assertRaw($this->fieldTestData->field->getLabel(), "First field's label is displayed.");
-    $this->assertSession()->responseContains($this->fieldTestData->field->getLabel(), "First field's label is displayed.");
     foreach ($values as $delta => $value) {
+      $this->assertRaw("$formatter_setting|{$value['value']}", "Value $delta is displayed, formatter settings are applied.");
-      $this->assertSession()->responseContains("$formatter_setting|{$value['value']}", "Value $delta is displayed, formatter settings are applied.");
     }
+    $this->assertRaw($this->fieldTestData->field_2->getLabel(), "Second field's label is displayed.");
-    $this->assertSession()->responseContains($this->fieldTestData->field_2->getLabel(), "Second field's label is displayed.");
     foreach ($values_2 as $delta => $value) {
+      $this->assertRaw("$formatter_setting_2|{$value['value']}", "Value $delta is displayed, formatter settings are applied.");
-      $this->assertSession()->responseContains("$formatter_setting_2|{$value['value']}", "Value $delta is displayed, formatter settings are applied.");
     }
 
     // Label hidden.
@@ -108,7 +108,7 @@
     foreach ($values as $delta => $value) {
       $expected_output .= "|$delta:{$value['value']}";
     }
+    $this->assertRaw($expected_output, "Multiple formatter: all values are displayed, formatter settings are applied.");
-    $this->assertSession()->responseContains($expected_output, "Multiple formatter: all values are displayed, formatter settings are applied.");
 
     // Test a formatter that uses hook_field_formatter_prepare_view().
     $entity = clone($entity_init);
@@ -124,7 +124,7 @@
     $this->render($content);
     foreach ($values as $delta => $value) {
       $expected = $formatter_setting . '|' . $value['value'] . '|' . ($value['value'] + 1);
+      $this->assertRaw($expected, "Value $delta is displayed, formatter settings are applied.");
-      $this->assertSession()->responseContains($expected, "Value $delta is displayed, formatter settings are applied.");
     }
 
     // TODO:
reverted:
--- b/core/modules/field/tests/src/Kernel/String/RawStringFormatterTest.php
+++ a/core/modules/field/tests/src/Kernel/String/RawStringFormatterTest.php
@@ -119,7 +119,7 @@
     // Verify that all HTML is escaped and newlines are retained.
     $this->renderEntityFields($entity, $this->display);
     $this->assertNoRaw($value);
+    $this->assertRaw(nl2br(Html::escape($value)));
-    $this->assertSession()->responseContains(nl2br(Html::escape($value)));
 
     // Verify the cache tags.
     $build = $entity->{$this->fieldName}->view();
reverted:
--- b/core/modules/field/tests/src/Kernel/String/StringFormatterTest.php
+++ a/core/modules/field/tests/src/Kernel/String/StringFormatterTest.php
@@ -129,7 +129,7 @@
     // Verify that all HTML is escaped and newlines are retained.
     $this->renderEntityFields($entity, $this->display);
     $this->assertNoRaw($value);
+    $this->assertRaw(nl2br(Html::escape($value)));
-    $this->assertSession()->responseContains(nl2br(Html::escape($value)));
 
     // Verify the cache tags.
     $build = $entity->{$this->fieldName}->view();
reverted:
--- b/core/modules/field/tests/src/Kernel/Timestamp/TimestampFormatterTest.php
+++ a/core/modules/field/tests/src/Kernel/Timestamp/TimestampFormatterTest.php
@@ -136,7 +136,7 @@
       $entity->{$this->fieldName}->value = $value;
 
       $this->renderEntityFields($entity, $this->display);
+      $this->assertRaw($expected);
-      $this->assertSession()->responseContains($expected);
     }
   }
 
@@ -173,7 +173,7 @@
       $entity->{$this->fieldName}->value = $value;
 
       $this->renderEntityFields($entity, $this->display);
+      $this->assertRaw($expected);
-      $this->assertSession()->responseContains($expected);
 
       // Test a timestamp in the future
       $value = $request_time + 87654321;
@@ -188,7 +188,7 @@
       $entity->{$this->fieldName}->value = $value;
 
       $this->renderEntityFields($entity, $this->display);
+      $this->assertRaw($expected);
-      $this->assertSession()->responseContains($expected);
     }
   }
 
reverted:
--- b/core/modules/filter/tests/src/Kernel/TextFormatElementFormTest.php
+++ a/core/modules/filter/tests/src/Kernel/TextFormatElementFormTest.php
@@ -118,9 +118,9 @@
     $output = $this->render($form);
     $this->setRawContent($output);
     $this->assertFieldByName('textformat[value]');
+    $this->assertRaw('<h4>Full HTML</h4>');
+    $this->assertRaw('<h4>Filtered HTML</h4>');
+    $this->assertRaw('<h4>Test format</h4>');
-    $this->assertSession()->responseContains('<h4>Full HTML</h4>');
-    $this->assertSession()->responseContains('<h4>Filtered HTML</h4>');
-    $this->assertSession()->responseContains('<h4>Test format</h4>');
     $this->assertNoPattern('|<h4[^>]*></h4>|', 'No empty H4 element found.');
   }
 
reverted:
--- b/core/modules/node/tests/src/Kernel/SummaryLengthTest.php
+++ a/core/modules/node/tests/src/Kernel/SummaryLengthTest.php
@@ -96,7 +96,7 @@
     // characters of the node body, so it should be included if the summary is
     // 600 characters long.
     $expected = 'What is a Drupalism?';
+    $this->assertRaw($expected);
-    $this->assertSession()->responseContains($expected);
 
     // Change the teaser length for "Basic page" content type.
     $display = \Drupal::service('entity_display.repository')
reverted:
--- b/core/modules/system/tests/src/Kernel/Theme/TwigFilterTest.php
+++ a/core/modules/system/tests/src/Kernel/Theme/TwigFilterTest.php
@@ -132,7 +132,7 @@
     ];
 
     foreach ($elements as $element) {
+      $this->assertRaw($element['expected'], $element['message']);
-      $this->assertSession()->responseContains($element['expected'], $element['message']);
     }
   }
 
reverted:
--- b/core/modules/system/tests/src/Kernel/Theme/TwigRawTest.php
+++ a/core/modules/system/tests/src/Kernel/Theme/TwigRawTest.php
@@ -28,7 +28,7 @@
     ];
     $rendered = \Drupal::service('renderer')->renderRoot($test);
     $this->setRawContent($rendered);
+    $this->assertRaw('<script>alert("This alert is real because I will put it through the raw filter!");</script>');
-    $this->assertSession()->responseContains('<script>alert("This alert is real because I will put it through the raw filter!");</script>');
   }
 
   /**
reverted:
--- b/core/modules/user/tests/src/Kernel/WhosOnlineBlockTest.php
+++ a/core/modules/user/tests/src/Kernel/WhosOnlineBlockTest.php
@@ -110,7 +110,7 @@
       ->getViewBuilder($entity->getEntityTypeId())
       ->view($entity, 'block');
     $this->setRawContent($this->renderer->renderRoot($output));
+    $this->assertRaw('2 users', 'Correct number of online users (2 users).');
-    $this->assertSession()->responseContains('2 users', 'Correct number of online users (2 users).');
     $this->assertText($user1->getAccountName(), 'Active user 1 found in online list.');
     $this->assertText($user2->getAccountName(), 'Active user 2 found in online list.');
     $this->assertNoText($user3->getAccountName(), 'Inactive user not found in online list.');
reverted:
--- b/core/modules/views/tests/src/Kernel/Handler/AreaResultTest.php
+++ a/core/modules/views/tests/src/Kernel/Handler/AreaResultTest.php
@@ -57,7 +57,7 @@
     $output = \Drupal::service('renderer')->renderRoot($output);
     $this->setRawContent($output);
     $this->assertText('start: 0 | end: 0 | total: 0 | label: test_area_result | per page: 0 | current page: 1 | current record count: 0 | page count: 1');
+    $this->assertRaw('<header>');
-    $this->assertSession()->responseContains('<header>');
 
     // Test that the area is not displayed if we have not checked the empty
     // checkbox.
reverted:
--- b/core/modules/views/tests/src/Kernel/ViewsHooksTest.php
+++ a/core/modules/views/tests/src/Kernel/ViewsHooksTest.php
@@ -113,7 +113,7 @@
     });
     $this->setRawContent((string) $element['output']['#markup']);
     $this->assertEscaped('<em>escaped</em>');
+    $this->assertRaw('<em>unescaped</em>');
-    $this->assertSession()->responseContains('<em>unescaped</em>');
   }
 
 }
reverted:
--- b/core/modules/workspaces/tests/src/Kernel/WorkspaceIntegrationTest.php
+++ a/core/modules/workspaces/tests/src/Kernel/WorkspaceIntegrationTest.php
@@ -786,7 +786,7 @@
       $this->setRawContent($output);
       foreach ($expected_values as $expected_entity_values) {
         if ($expected_entity_values[$entity_keys['published']] === TRUE && $expected_entity_values['default_revision'] === TRUE) {
+          $this->assertRaw($expected_entity_values[$entity_keys['label']]);
-          $this->assertSession()->responseContains($expected_entity_values[$entity_keys['label']]);
         }
         // Node 4 will always appear in the 'stage' workspace because it has
         // both an unpublished revision as well as a published one.
reverted:
--- b/core/tests/Drupal/FunctionalTests/Bootstrap/UncaughtExceptionTest.php
+++ a/core/tests/Drupal/FunctionalTests/Bootstrap/UncaughtExceptionTest.php
@@ -117,10 +117,10 @@
     $this->drupalGet('error-test/generate-fatals');
     $this->assertResponse(500);
     $message = new FormattableMarkup('%type: @message in %function (line ', $fatal_error);
+    $this->assertRaw((string) $message);
+    $this->assertRaw('<pre class="backtrace">');
-    $this->assertSession()->responseContains((string) $message);
-    $this->assertSession()->responseContains('<pre class="backtrace">');
     // Ensure we are escaping but not double escaping.
+    $this->assertRaw('&#039;');
-    $this->assertSession()->responseContains('&#039;');
     $this->assertNoRaw('&amp;#039;');
   }
 
@@ -160,8 +160,8 @@
     $this->drupalGet('broken-service-class');
     $this->assertResponse(500);
 
+    $this->assertRaw('The website encountered an unexpected error.');
+    $this->assertRaw($this->expectedExceptionMessage);
-    $this->assertSession()->responseContains('The website encountered an unexpected error.');
-    $this->assertSession()->responseContains($this->expectedExceptionMessage);
     $this->assertErrorLogged($this->expectedExceptionMessage);
   }
 
@@ -202,7 +202,7 @@
     $this->drupalGet('');
     $this->assertResponse(500);
 
+    $this->assertRaw($this->expectedExceptionMessage);
-    $this->assertSession()->responseContains($this->expectedExceptionMessage);
     $this->assertErrorLogged($this->expectedExceptionMessage);
   }
 
@@ -222,8 +222,8 @@
     $this->drupalGet('');
     $this->assertResponse(500);
 
+    $this->assertRaw('The website encountered an unexpected error');
+    $this->assertRaw($this->expectedExceptionMessage);
-    $this->assertSession()->responseContains('The website encountered an unexpected error');
-    $this->assertSession()->responseContains($this->expectedExceptionMessage);
     $this->assertErrorLogged($this->expectedExceptionMessage);
   }
 
@@ -258,7 +258,7 @@
 
     $this->drupalGet('');
     $this->assertResponse(500);
+    $this->assertRaw('DatabaseAccessDeniedException');
-    $this->assertSession()->responseContains('DatabaseAccessDeniedException');
     $this->assertErrorLogged($this->expectedExceptionMessage);
   }
 
@@ -275,7 +275,7 @@
     $this->drupalGet('');
     $this->assertResponse(500);
     $this->assertText('The website encountered an unexpected error. Please try again later.');
+    $this->assertRaw($this->expectedExceptionMessage);
-    $this->assertSession()->responseContains($this->expectedExceptionMessage);
 
     // Find fatal error logged to the error.log
     $errors = file(\Drupal::root() . '/' . $this->siteDirectory . '/error.log');
reverted:
--- b/core/tests/Drupal/KernelTests/Core/Render/Element/TableTest.php
+++ a/core/tests/Drupal/KernelTests/Core/Render/Element/TableTest.php
@@ -34,7 +34,7 @@
     // Make sure tableheader.js was attached.
     $tableheader = $this->xpath("//script[contains(@src, 'tableheader.js')]");
     $this->assertCount(1, $tableheader);
+    $this->assertRaw('sticky-enabled');
-    $this->assertSession()->responseContains('sticky-enabled');
   }
 
   /**
@@ -87,8 +87,8 @@
 
     $this->render($table);
     $this->removeWhiteSpace();
+    $this->assertRaw('<thead><tr><th>Header 1</th><th colspan="2">Header 2</th></tr>', 'Table header found.');
+    $this->assertRaw('<tr class="odd"><td colspan="3" class="empty message">Empty row.</td>', 'Colspan on #empty row found.');
-    $this->assertSession()->responseContains('<thead><tr><th>Header 1</th><th colspan="2">Header 2</th></tr>', 'Table header found.');
-    $this->assertSession()->responseContains('<tr class="odd"><td colspan="3" class="empty message">Empty row.</td>', 'Colspan on #empty row found.');
   }
 
   /**
@@ -129,7 +129,7 @@
 
     $this->render($table);
     $this->removeWhiteSpace();
+    $this->assertRaw('<tfoot><tr><td>1</td></tr><tr><td>Foo</td></tr></tfoot>', 'Table footer found.');
-    $this->assertSession()->responseContains('<tfoot><tr><td>1</td></tr><tr><td>Foo</td></tr></tfoot>', 'Table footer found.');
   }
 
   /**
@@ -149,7 +149,7 @@
     ];
     $this->render($table);
     $this->removeWhiteSpace();
+    $this->assertRaw('<th>1</th><td>1</td><td>1</td>', 'The th and td tags was printed correctly.');
-    $this->assertSession()->responseContains('<th>1</th><td>1</td><td>1</td>', 'The th and td tags was printed correctly.');
   }
 
   /**
@@ -165,7 +165,7 @@
       '#responsive' => TRUE,
     ];
     $this->render($table);
+    $this->assertRaw('responsive-enabled', 'The responsive-enabled class was printed correctly.');
-    $this->assertSession()->responseContains('responsive-enabled', 'The responsive-enabled class was printed correctly.');
   }
 
   /**
@@ -218,12 +218,12 @@
       '#responsive' => TRUE,
     ];
     $this->render($table);
+    $this->assertRaw('<th class="priority-medium">1</th>', 'Header 1: the priority-medium class was applied correctly.');
+    $this->assertRaw('<th class="priority-low">2</th>', 'Header 2: the priority-low class was applied correctly.');
+    $this->assertRaw('<th>3</th>', 'Header 3: no priority classes were applied.');
+    $this->assertRaw('<td class="priority-medium">4</td>', 'Cell 1: the priority-medium class was applied correctly.');
+    $this->assertRaw('<td class="priority-low">5</td>', 'Cell 2: the priority-low class was applied correctly.');
+    $this->assertRaw('<td>6</td>', 'Cell 3: no priority classes were applied.');
-    $this->assertSession()->responseContains('<th class="priority-medium">1</th>', 'Header 1: the priority-medium class was applied correctly.');
-    $this->assertSession()->responseContains('<th class="priority-low">2</th>', 'Header 2: the priority-low class was applied correctly.');
-    $this->assertSession()->responseContains('<th>3</th>', 'Header 3: no priority classes were applied.');
-    $this->assertSession()->responseContains('<td class="priority-medium">4</td>', 'Cell 1: the priority-medium class was applied correctly.');
-    $this->assertSession()->responseContains('<td class="priority-low">5</td>', 'Cell 2: the priority-low class was applied correctly.');
-    $this->assertSession()->responseContains('<td>6</td>', 'Cell 3: no priority classes were applied.');
   }
 
   /**
@@ -254,7 +254,7 @@
     ];
     $this->render($table);
     $this->removeWhiteSpace();
+    $this->assertRaw('<thead><tr><th>one</th><th>two</th><th><b>three</b></th></tr>', 'Table header found.');
-    $this->assertSession()->responseContains('<thead><tr><th>one</th><th>two</th><th><b>three</b></th></tr>', 'Table header found.');
   }
 
   /**
@@ -294,8 +294,8 @@
     ];
     $this->render($table);
     $this->removeWhiteSpace();
+    $this->assertRaw('<tbody><tr><td>1-one</td><td>1-two</td><td>1-three</td></tr>', 'Table row 1 found.');
+    $this->assertRaw('<tr><td>2-one</td><td>2-two</td><td><b>2-three</b></td></tr></tbody>', 'Table row 2 found.');
-    $this->assertSession()->responseContains('<tbody><tr><td>1-one</td><td>1-two</td><td>1-three</td></tr>', 'Table row 1 found.');
-    $this->assertSession()->responseContains('<tr><td>2-one</td><td>2-two</td><td><b>2-three</b></td></tr></tbody>', 'Table row 2 found.');
   }
 
   /**
@@ -305,7 +305,7 @@
     $form = \Drupal::formBuilder()->getForm('\Drupal\form_test\Form\FormTestTableForm');
     $this->render($form);
     $this->assertEscaped('Update <em>kitten</em>');
+    $this->assertRaw('Update my favorite fruit is <strong>bananas</strong>');
-    $this->assertSession()->responseContains('Update my favorite fruit is <strong>bananas</strong>');
   }
 
 }
reverted:
--- b/core/tests/Drupal/KernelTests/Core/Routing/ExceptionHandlingTest.php
+++ a/core/tests/Drupal/KernelTests/Core/Routing/ExceptionHandlingTest.php
@@ -186,7 +186,7 @@
     // Test message is properly escaped, and that the unescaped string is not
     // output at all.
     $this->setRawContent($response->getContent());
+    $this->assertRaw(Html::escape('Escaped content: <p> <br> <h3>'));
-    $this->assertSession()->responseContains(Html::escape('Escaped content: <p> <br> <h3>'));
     $this->assertNoRaw('<p> <br> <h3>');
 
     $string = '<script>alert(123);</script>';
reverted:
--- b/core/tests/Drupal/KernelTests/Core/Theme/ImageTest.php
+++ a/core/tests/Drupal/KernelTests/Core/Theme/ImageTest.php
@@ -60,7 +60,7 @@
     $this->render($image);
 
     // Make sure sizes is set.
+    $this->assertRaw($sizes, 'Sizes is set correctly.');
-    $this->assertSession()->responseContains($sizes, 'Sizes is set correctly.');
   }
 
   /**
@@ -79,7 +79,7 @@
     $this->render($image);
 
     // Make sure the src attribute has the correct value.
+    $this->assertRaw(file_url_transform_relative(file_create_url($image['#uri'])), 'Correct output for an image with the src attribute.');
-    $this->assertSession()->responseContains(file_url_transform_relative(file_create_url($image['#uri'])), 'Correct output for an image with the src attribute.');
   }
 
   /**
@@ -107,7 +107,7 @@
     $this->render($image);
 
     // Make sure the srcset attribute has the correct value.
+    $this->assertRaw(file_url_transform_relative(file_create_url($this->testImages[0])) . ' 1x, ' . file_url_transform_relative(file_create_url($this->testImages[1])) . ' 2x', 'Correct output for image with srcset attribute and multipliers.');
-    $this->assertSession()->responseContains(file_url_transform_relative(file_create_url($this->testImages[0])) . ' 1x, ' . file_url_transform_relative(file_create_url($this->testImages[1])) . ' 2x', 'Correct output for image with srcset attribute and multipliers.');
   }
 
   /**
@@ -139,7 +139,7 @@
     $this->render($image);
 
     // Make sure the srcset attribute has the correct value.
+    $this->assertRaw(file_url_transform_relative(file_create_url($this->testImages[0])) . ' ' . $widths[0] . ', ' . file_url_transform_relative(file_create_url($this->testImages[1])) . ' ' . $widths[1], 'Correct output for image with srcset attribute and width descriptors.');
-    $this->assertSession()->responseContains(file_url_transform_relative(file_create_url($this->testImages[0])) . ' ' . $widths[0] . ', ' . file_url_transform_relative(file_create_url($this->testImages[1])) . ' ' . $widths[1], 'Correct output for image with srcset attribute and width descriptors.');
   }
 
 }
reverted:
--- b/core/tests/Drupal/KernelTests/Core/Theme/MessageTest.php
+++ a/core/tests/Drupal/KernelTests/Core/Theme/MessageTest.php
@@ -30,8 +30,8 @@
       '#type' => 'status_messages',
     ];
     $this->render($messages);
+    $this->assertRaw('messages messages--error');
+    $this->assertRaw('messages messages--status');
-    $this->assertSession()->responseContains('messages messages--error');
-    $this->assertSession()->responseContains('messages messages--status');
     // Tests display of only one type of messages.
     \Drupal::messenger()->addError('An error occurred');
     $messages = [
@@ -39,7 +39,7 @@
       '#display' => 'error',
     ];
     $this->render($messages);
+    $this->assertRaw('messages messages--error');
-    $this->assertSession()->responseContains('messages messages--error');
   }
 
 }
reverted:
--- b/core/tests/Drupal/KernelTests/Core/Url/LinkGenerationTest.php
+++ a/core/tests/Drupal/KernelTests/Core/Url/LinkGenerationTest.php
@@ -29,7 +29,7 @@
     $this->setRawContent($link);
     $this->assertInstanceOf(MarkupInterface::class, $link);
     // Ensure the content of the link is not escaped.
+    $this->assertRaw('<em>link with markup</em>');
-    $this->assertSession()->responseContains('<em>link with markup</em>');
 
     // Test just adding text to an already safe string.
     \Drupal::state()->set('link_generation_test_link_alter', TRUE);
@@ -49,7 +49,7 @@
     $this->setRawContent($link);
     $this->assertInstanceOf(MarkupInterface::class, $link);
     // Ensure the content of the link is escaped.
+    $this->assertRaw('<em>link with markup</em> <strong>Test!</strong>');
-    $this->assertSession()->responseContains('<em>link with markup</em> <strong>Test!</strong>');
 
     // Test passing an unsafe string to t().
     $link = $renderer->executeInRenderContext(new RenderContext(), function () use ($url) {
@@ -59,7 +59,7 @@
     $this->assertInstanceOf(MarkupInterface::class, $link);
     // Ensure the content of the link is escaped.
     $this->assertEscaped('<em>link with markup</em>');
+    $this->assertRaw('<strong>Test!</strong>');
-    $this->assertSession()->responseContains('<strong>Test!</strong>');
   }
 
 }