diff --git a/core/modules/content_moderation/tests/src/Functional/ModerationActionsTest.php b/core/modules/content_moderation/tests/src/Functional/ModerationActionsTest.php
index 5d3a98965b..9632540f98 100644
--- a/core/modules/content_moderation/tests/src/Functional/ModerationActionsTest.php
+++ b/core/modules/content_moderation/tests/src/Functional/ModerationActionsTest.php
@@ -31,7 +31,7 @@ class ModerationActionsTest extends BrowserTestBase {
   /**
    * {@inheritdoc}
    */
-  protected $defaultTheme = 'classy';
+  protected $defaultTheme = 'stark';
 
   /**
    * {@inheritdoc}
@@ -81,16 +81,14 @@ public function testNodeStatusActions($action, $bundle, $warning_appears, $start
 
     if ($warning_appears) {
       if ($action == 'node_publish_action') {
-        $this->assertSession()
-          ->elementContains('css', '.messages--warning', node_get_type_label($node) . ' content items were skipped as they are under moderation and may not be directly published.');
+        $this->assertSession()->statusMessageContains(node_get_type_label($node) . ' content items were skipped as they are under moderation and may not be directly published.', 'warning');
       }
       else {
-        $this->assertSession()
-          ->elementContains('css', '.messages--warning', node_get_type_label($node) . ' content items were skipped as they are under moderation and may not be directly unpublished.');
+        $this->assertSession()->statusMessageContains(node_get_type_label($node) . ' content items were skipped as they are under moderation and may not be directly unpublished.', 'warning');
       }
     }
     else {
-      $this->assertSession()->elementNotExists('css', '.messages--warning');
+      $this->assertSession()->statusMessageNotExists('warning');
     }
 
     // Ensure after the action has run, the node matches the expected status.
diff --git a/core/modules/file/tests/src/FunctionalJavascript/MaximumFileSizeExceededUploadTest.php b/core/modules/file/tests/src/FunctionalJavascript/MaximumFileSizeExceededUploadTest.php
index 6b7b7f894e..25361b4674 100644
--- a/core/modules/file/tests/src/FunctionalJavascript/MaximumFileSizeExceededUploadTest.php
+++ b/core/modules/file/tests/src/FunctionalJavascript/MaximumFileSizeExceededUploadTest.php
@@ -108,17 +108,16 @@ public function testUploadFileExceedingMaximumFileSize() {
     $page->attachFileToField("files[field_file_0]", $this->fileSystem->realpath($invalid_file));
 
     // An error message should appear informing the user that the file exceeded
-    // the maximum file size.
-    $this->assertSession()->waitForElement('css', '.messages--error');
-    // The error message includes the actual file size limit which depends on
-    // the current environment, so we check for a part of the message.
-    $this->assertSession()->pageTextContains('An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size');
+    // the maximum file size. The error message includes the actual file size
+    // limit which depends on the current environment, so we check for a part
+    // of the message.
+    $this->assertSession()->statusMessageContainsAfterWait('An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size', 'error');
 
     // Now upload a valid file and check that the error message disappears.
     $valid_file = $this->generateFile('not_exceeding_post_max_size', 8, 8);
     $page->attachFileToField("files[field_file_0]", $this->fileSystem->realpath($valid_file));
     $this->assertSession()->waitForElement('named', ['id_or_name', 'field_file_0_remove_button']);
-    $this->assertSession()->elementNotExists('css', '.messages--error');
+    $this->assertSession()->statusMessageNotExistsAfterWait('error');
   }
 
 }
diff --git a/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php b/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
index 10d812e26e..03f44fec4b 100644
--- a/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
+++ b/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
@@ -146,6 +146,27 @@ public function messengerServiceTest() {
     return [];
   }
 
+  /**
+   * Sets messages for testing the WebAssert methods related to messages.
+   *
+   * @return array
+   *   Empty array, we just need the messages.
+   */
+  public function statusMessagesForAssertions(): array {
+    // Add a simple message of each type.
+    $this->messenger->addMessage('My Status Message', 'status');
+    $this->messenger->addMessage('My Error Message', 'error');
+    $this->messenger->addMessage('My Warning Message', 'warning');
+
+    // Add messages with special characters and/or markup.
+    $this->messenger->addStatus('This has " in the middle');
+    $this->messenger->addStatus('This has \' in the middle');
+    $this->messenger->addStatus('<em>This<span>markup will be</span> escaped</em>.');
+    $this->messenger->addStatus('Peaches & cream');
+
+    return [];
+  }
+
   /**
    * Controller to return $_GET['destination'] for testing.
    *
diff --git a/core/modules/system/tests/modules/system_test/system_test.routing.yml b/core/modules/system/tests/modules/system_test/system_test.routing.yml
index 800a289c7c..59b5d9116e 100644
--- a/core/modules/system/tests/modules/system_test/system_test.routing.yml
+++ b/core/modules/system/tests/modules/system_test/system_test.routing.yml
@@ -21,6 +21,14 @@ system_test.messenger_service:
   requirements:
     _access: 'TRUE'
 
+system_test.status_messages_for_assertions:
+  path: '/system-test/status-messages-for-assertions'
+  defaults:
+    _title: 'Set various message to test status message assertion methods'
+    _controller: '\Drupal\system_test\Controller\SystemTestController::statusMessagesForAssertions'
+  requirements:
+    _access: 'TRUE'
+
 system_test.main_content_fallback:
   path: '/system-test/main-content-fallback'
   defaults:
diff --git a/core/modules/system/tests/src/Functional/Bootstrap/DrupalMessengerServiceTest.php b/core/modules/system/tests/src/Functional/Bootstrap/DrupalMessengerServiceTest.php
index 72dcca53be..a0d51126a2 100644
--- a/core/modules/system/tests/src/Functional/Bootstrap/DrupalMessengerServiceTest.php
+++ b/core/modules/system/tests/src/Functional/Bootstrap/DrupalMessengerServiceTest.php
@@ -4,6 +4,7 @@
 
 use Drupal\Core\Url;
 use Drupal\Tests\BrowserTestBase;
+use PHPUnit\Framework\AssertionFailedError;
 
 /**
  * Tests the Messenger service.
@@ -61,4 +62,91 @@ public function testDrupalMessengerService() {
     $assert->pageTextContains('system_test_preinstall_module called');
   }
 
+  /**
+   * Tests assertion methods in WebAssert related to status messages.
+   */
+  public function testStatusMessageAssertions(): void {
+    $this->drupalGet(Url::fromRoute('system_test.status_messages_for_assertions'));
+
+    // Use the simple messages to test basic functionality.
+    // Test WebAssert::statusMessagesExists().
+    $this->assertSession()->statusMessageExists();
+    $this->assertSession()->statusMessageExists('status');
+    $this->assertSession()->statusMessageExists('error');
+    $this->assertSession()->statusMessageExists('warning');
+
+    // WebAssert::statusMessageContains().
+    $this->assertSession()->statusMessageContains('My Status Message');
+    $this->assertSession()->statusMessageContains('My Error Message');
+    $this->assertSession()->statusMessageContains('My Warning Message');
+    // Test partial match.
+    $this->assertSession()->statusMessageContains('My Status');
+    // Test with second arg.
+    $this->assertSession()->statusMessageContains('My Status Message', 'status');
+    $this->assertSession()->statusMessageContains('My Error Message', 'error');
+    $this->assertSession()->statusMessageContains('My Warning Message', 'warning');
+
+    // Test WebAssert::statusMessageNotContains().
+    $this->assertSession()->statusMessageNotContains('My Status Message is fake');
+    $this->assertSession()->statusMessageNotContains('My Status Message is fake', 'status');
+    $this->assertSession()->statusMessageNotContains('My Error Message', 'status');
+    $this->assertSession()->statusMessageNotContains('My Status Message', 'error');
+
+    // Check that special characters get handled correctly.
+    $this->assertSession()->statusMessageContains('This has " in the middle');
+    $this->assertSession()->statusMessageContains('This has \' in the middle');
+    $this->assertSession()->statusMessageContains('<em>This<span>markup will be</span> escaped</em>');
+    $this->assertSession()->statusMessageContains('Peaches & cream');
+    $this->assertSession()->statusMessageNotContains('Peaches &amp; cream');
+
+    // Go to a new route that only has messages of type 'status'.
+    $this->drupalGet(Url::fromRoute('system_test.messenger_service'));
+    // Test WebAssert::statusMessageNotExists().
+    $this->assertSession()->statusMessageNotExists('error');
+    $this->assertSession()->statusMessageNotExists('warning');
+
+    // Perform a few assertions that should fail. We can only call
+    // TestCase::expectException() once per test, so we make a few
+    // try/catch blocks.
+    $expected_failure_occurred = FALSE;
+    try {
+      $this->assertSession()->statusMessageContains('This message is not real');
+    }
+    catch (AssertionFailedError $e) {
+      $expected_failure_occurred = TRUE;
+    }
+    $this->assertTrue($expected_failure_occurred);
+
+    $expected_failure_occurred = FALSE;
+    try {
+      $this->assertSession()->statusMessageNotContains('markup');
+    }
+    catch (AssertionFailedError $e) {
+      $expected_failure_occurred = TRUE;
+    }
+    $this->assertTrue($expected_failure_occurred);
+
+    $expected_failure_occurred = FALSE;
+    try {
+      $this->assertSession()->statusMessageExists('error');
+    }
+    catch (AssertionFailedError $e) {
+      $expected_failure_occurred = TRUE;
+    }
+    $this->assertTrue($expected_failure_occurred);
+
+    $expected_failure_occurred = FALSE;
+    try {
+      $this->assertSession()->statusMessageNotExists();
+    }
+    catch (AssertionFailedError $e) {
+      $expected_failure_occurred = TRUE;
+    }
+    $this->assertTrue($expected_failure_occurred);
+
+    // Tests passing a bad status type.
+    $this->expectException(\InvalidArgumentException::class);
+    $this->assertSession()->statusMessageExists('not a valid type');
+  }
+
 }
diff --git a/core/tests/Drupal/FunctionalJavascriptTests/Ajax/MessageCommandTest.php b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/MessageCommandTest.php
index 03c6b3af15..9b8a6f3468 100644
--- a/core/tests/Drupal/FunctionalJavascriptTests/Ajax/MessageCommandTest.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/MessageCommandTest.php
@@ -3,6 +3,7 @@
 namespace Drupal\FunctionalJavascriptTests\Ajax;
 
 use Drupal\FunctionalJavascriptTests\WebDriverTestBase;
+use PHPUnit\Framework\ExpectationFailedException;
 
 /**
  * Tests adding messages via AJAX command.
@@ -76,6 +77,94 @@ public function testMessageCommand() {
     }
   }
 
+  /**
+   * Tests methods in JsWebAssert related to status messages.
+   */
+  public function testJsStatusMessageAssertions(): void {
+    $page = $this->getSession()->getPage();
+
+    $this->drupalGet('ajax-test/message');
+
+    $page->pressButton('Make Message In Default Location');
+    $this->assertSession()->statusMessageContainsAfterWait('I am a message in the default location.');
+
+    $page->pressButton('Make Message In Alternate Location');
+    $this->assertSession()->statusMessageContainsAfterWait('I am a message in an alternate location.', 'status');
+
+    $page->pressButton('Make Warning Message');
+    $this->assertSession()->statusMessageContainsAfterWait('I am a warning message in the default location.', 'warning');
+
+    // Reload and test some negative assertions.
+    $this->drupalGet('ajax-test/message');
+
+    $page->pressButton('Make Message In Default Location');
+    // Use message that is not on page.
+    $this->assertSession()->statusMessageNotContainsAfterWait('This is not a real message');
+
+    $page->pressButton('Make Message In Alternate Location');
+    // Use message that exists but has the wrong type.
+    $this->assertSession()->statusMessageNotContainsAfterWait('I am a message in an alternate location.', 'warning');
+
+    // Test partial match.
+    $page->pressButton('Make Warning Message');
+    $this->assertSession()->statusMessageContainsAfterWait('I am a warning');
+
+    // One more reload to try with different arg combinations.
+    $this->drupalGet('ajax-test/message');
+
+    $page->pressButton('Make Message In Default Location');
+    $this->assertSession()->statusMessageExistsAfterWait();
+
+    $page->pressButton('Make Message In Alternate Location');
+    $this->assertSession()->statusMessageNotExistsAfterWait('error');
+
+    $page->pressButton('Make Warning Message');
+    $this->assertSession()->statusMessageExistsAfterWait('warning');
+
+    // Perform a few assertions that should fail. We can only call
+    // TestCase::expectException() once per test, so we make a few
+    // try/catch blocks.
+    $expected_failure_occurred = FALSE;
+    try {
+      $this->assertSession()->statusMessageContainsAfterWait('Not a real message');
+    }
+    catch (ExpectationFailedException $e) {
+      $expected_failure_occurred = TRUE;
+    }
+    $this->assertTrue($expected_failure_occurred);
+
+    $expected_failure_occurred = FALSE;
+    try {
+      $this->assertSession()->statusMessageNotContainsAfterWait('I am a warning');
+    }
+    catch (ExpectationFailedException $e) {
+      $expected_failure_occurred = TRUE;
+    }
+    $this->assertTrue($expected_failure_occurred);
+
+    $expected_failure_occurred = FALSE;
+    try {
+      $this->assertSession()->statusMessageExistsAfterWait('error');
+    }
+    catch (ExpectationFailedException $e) {
+      $expected_failure_occurred = TRUE;
+    }
+    $this->assertTrue($expected_failure_occurred);
+
+    $expected_failure_occurred = FALSE;
+    try {
+      $this->assertSession()->statusMessageNotExistsAfterWait('warning');
+    }
+    catch (ExpectationFailedException $e) {
+      $expected_failure_occurred = TRUE;
+    }
+    $this->assertTrue($expected_failure_occurred);
+
+    // Tests passing a bad status type.
+    $this->expectException(\InvalidArgumentException::class);
+    $this->assertSession()->statusMessageExistsAfterWait('not a valid type');
+  }
+
   /**
    * Asserts that a message of the expected type appears.
    *
diff --git a/core/tests/Drupal/FunctionalJavascriptTests/JSWebAssert.php b/core/tests/Drupal/FunctionalJavascriptTests/JSWebAssert.php
index e9e1d9cdec..f3dd65b9be 100644
--- a/core/tests/Drupal/FunctionalJavascriptTests/JSWebAssert.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/JSWebAssert.php
@@ -8,6 +8,9 @@
 use Behat\Mink\Exception\ElementNotFoundException;
 use Behat\Mink\Exception\UnsupportedDriverActionException;
 use Drupal\Tests\WebAssert;
+use PHPUnit\Framework\Assert;
+use PHPUnit\Framework\Constraint\IsNull;
+use PHPUnit\Framework\Constraint\LogicalNot;
 use WebDriver\Exception;
 use WebDriver\Exception\CurlExec;
 
@@ -523,4 +526,137 @@ public static function isExceptionNotClickable(Exception $exception): bool {
     return (bool) preg_match('/not (clickable|interactable|visible)/', $exception->getMessage());
   }
 
+  /**
+   * Asserts that a status message exists after wait.
+   *
+   * @param string|null $type
+   *   The optional message type: status, error, or warning.
+   */
+  public function statusMessageExistsAfterWait(string $type = NULL): void {
+    $selector = $this->buildJavascriptStatusMessageSelector('', $type);
+    $status_message_element = $this->waitForElement('xpath', $selector);
+    if ($type) {
+      $failure_message = sprintf('A status message of type "%s" does not appear on this page, but it should.', $type);
+    }
+    else {
+      $failure_message = 'A status message does not appear on this page, but it should.';
+    }
+    // There is no Assert::isNotNull() method, so we make our own constraint.
+    $constraint = new LogicalNot(new IsNull());
+    Assert::assertThat($status_message_element, $constraint, $failure_message);
+  }
+
+  /**
+   * Asserts that a status message does not exist after wait.
+   *
+   * @param string|null $type
+   *   The optional message type: status, error, or warning.
+   */
+  public function statusMessageNotExistsAfterWait(string $type = NULL): void {
+    $selector = $this->buildJavascriptStatusMessageSelector('', $type);
+    $status_message_element = $this->waitForElement('xpath', $selector);
+    if ($type) {
+      $failure_message = sprintf('A status message of type "%s" appears on this page, but it should not.', $type);
+    }
+    else {
+      $failure_message = 'A status message appears on this page, but it should not.';
+    }
+    Assert::assertThat($status_message_element, Assert::isNull(), $failure_message);
+  }
+
+  /**
+   * Asserts that a status message containing given string exists after wait.
+   *
+   * @param string $message
+   *   The partial message to assert.
+   * @param string|null $type
+   *   The optional message type: status, error, or warning.
+   */
+  public function statusMessageContainsAfterWait(string $message, string $type = NULL): void {
+    $selector = $this->buildJavascriptStatusMessageSelector($message, $type);
+    $status_message_element = $this->waitForElement('xpath', $selector);
+    if ($type) {
+      $failure_message = sprintf('A status message of type "%s" containing "%s" does not appear on this page, but it should.', $type, $message);
+    }
+    else {
+      $failure_message = sprintf('A status message containing "%s" does not appear on this page, but it should.', $type, $message);
+    }
+    // There is no Assert::isNotNull() method, so we make our own constraint.
+    $constraint = new LogicalNot(new IsNull());
+    Assert::assertThat($status_message_element, $constraint, $failure_message);
+  }
+
+  /**
+   * Asserts that no status message containing given string exists after wait.
+   *
+   * @param string $message
+   *   The partial message to assert.
+   * @param string|null $type
+   *   The optional message type: status, error, or warning.
+   */
+  public function statusMessageNotContainsAfterWait(string $message, string $type = NULL): void {
+    $selector = $this->buildJavascriptStatusMessageSelector($message, $type);
+    $status_message_element = $this->waitForElement('xpath', $selector);
+    if ($type) {
+      $failure_message = sprintf('A status message of type "%s" containing "%s" appears on this page, but it should not.', $type, $message);
+    }
+    else {
+      $failure_message = sprintf('A status message containing "%s" appears on this page, but it should not.', $message);
+    }
+    Assert::assertThat($status_message_element, Assert::isNull(), $failure_message);
+  }
+
+  /**
+   * Builds a xpath selector for a message with given type and text.
+   *
+   * The selector is designed to work with the Drupal.theme.message
+   * template defined in message.js in addition to status-messages.html.twig
+   * in the system module.
+   *
+   * @param string|null $message
+   *   The optional message or partial message to assert.
+   * @param string|null $type
+   *   The optional message type: status, error, or warning.
+   *
+   * @return string
+   *   The xpath selector for the message.
+   *
+   * @throws \InvalidArgumentException
+   *   Thrown when $type is not an allowed type.
+   */
+  private function buildJavascriptStatusMessageSelector(string $message = NULL, string $type = NULL): string {
+    $allowed_types = [
+      'status',
+      'error',
+      'warning',
+      NULL,
+    ];
+    if (!in_array($type, $allowed_types, TRUE)) {
+      throw new \InvalidArgumentException(sprintf("If a status message type is specified, the allowed values are 'status', 'error', 'warning'. The value provided was '%s'.", $type));
+    }
+
+    if ($type) {
+      $class = 'messages--' . $type;
+    }
+    else {
+      $class = 'messages__wrapper';
+    }
+
+    if ($message) {
+      $js_selector = $this->buildXPathQuery('//div[contains(@class, :class) and contains(., :message)]', [
+        ':class' => $class,
+        ':message' => $message,
+      ]);
+    }
+    else {
+      $js_selector = $this->buildXPathQuery('//div[contains(@class, :class)]', [
+        ':class' => $class,
+      ]);
+    }
+
+    // We select based on WebAssert::buildStatusMessageSelector() or the
+    // js_selector we have just built.
+    return $this->buildStatusMessageSelector($message, $type) . ' | ' . $js_selector;
+  }
+
 }
diff --git a/core/tests/Drupal/Tests/WebAssert.php b/core/tests/Drupal/Tests/WebAssert.php
index a5d80a6e3c..4e2c0c0707 100644
--- a/core/tests/Drupal/Tests/WebAssert.php
+++ b/core/tests/Drupal/Tests/WebAssert.php
@@ -1125,4 +1125,136 @@ public function checkboxNotChecked($field, TraversableElement $container = NULL)
     return parent::checkboxNotChecked($field, $container);
   }
 
+  /**
+   * Asserts that a status message exists.
+   *
+   * @param string|null $type
+   *   The optional message type: status, error, or warning.
+   */
+  public function statusMessageExists(string $type = NULL): void {
+    $selector = $this->buildStatusMessageSelector('', $type);
+    try {
+      $this->elementExists('xpath', $selector);
+    }
+    catch (ExpectationException $e) {
+      Assert::fail($e->getMessage());
+    }
+  }
+
+  /**
+   * Asserts that a status message does not exist.
+   *
+   * @param string|null $type
+   *   The optional message type: status, error, or warning.
+   */
+  public function statusMessageNotExists(string $type = NULL): void {
+    $selector = $this->buildStatusMessageSelector('', $type);
+    try {
+      $this->elementNotExists('xpath', $selector);
+    }
+    catch (ExpectationException $e) {
+      Assert::fail($e->getMessage());
+    }
+  }
+
+  /**
+   * Asserts that a status message containing a given string exists.
+   *
+   * @param string $message
+   *   The partial message to assert.
+   * @param string|null $type
+   *   The optional message type: status, error, or warning.
+   */
+  public function statusMessageContains(string $message, string $type = NULL): void {
+    $selector = $this->buildStatusMessageSelector($message, $type);
+    try {
+      $this->elementExists('xpath', $selector);
+    }
+    catch (ExpectationException $e) {
+      Assert::fail($e->getMessage());
+    }
+  }
+
+  /**
+   * Asserts that a status message containing a given string does not exist.
+   *
+   * @param string $message
+   *   The partial message to assert.
+   * @param string|null $type
+   *   The optional message type: status, error, or warning.
+   */
+  public function statusMessageNotContains(string $message, string $type = NULL): void {
+    $selector = $this->buildStatusMessageSelector($message, $type);
+    try {
+      $this->elementNotExists('xpath', $selector);
+    }
+    catch (ExpectationException $e) {
+      Assert::fail($e->getMessage());
+    }
+  }
+
+  /**
+   * Builds a xpath selector for a message with given type and text.
+   *
+   * The selector is designed to work with the status-messages.html.twig
+   * template in the system module.
+   *
+   * See Drupal\Core\Render\Element\StatusMessages for aria label definition.
+   *
+   * @param string|null $message
+   *   The optional message or partial message to assert.
+   * @param string|null $type
+   *   The optional message type: status, error, or warning.
+   *
+   * @return string
+   *   The xpath selector for the message.
+   *
+   * @throws \InvalidArgumentException
+   *   Thrown when $type is not an allowed type.
+   */
+  protected function buildStatusMessageSelector(string $message = NULL, string $type = NULL): string {
+    $allowed_types = [
+      'status',
+      'error',
+      'warning',
+      NULL,
+    ];
+    if (!in_array($type, $allowed_types, TRUE)) {
+      throw new \InvalidArgumentException(sprintf("If a status message type is specified, the allowed values are 'status', 'error', 'warning'. The value provided was '%s'.", $type));
+    }
+    $selector = '//div[@data-drupal-messages]';
+    $aria_label = NULL;
+    switch ($type) {
+      case 'status':
+        $aria_label = 'Status message';
+        break;
+
+      case 'error':
+        $aria_label = 'Error message';
+        break;
+
+      case 'warning':
+        $aria_label = 'Warning message';
+    }
+
+    if ($message && $aria_label) {
+      $selector = $this->buildXPathQuery($selector . '//div[contains(@aria-label, :aria_label) and contains(., :message)]', [
+        ':aria_label' => $aria_label,
+        ':message' => $message,
+      ]);
+    }
+    elseif ($message) {
+      $selector = $this->buildXPathQuery($selector . '//div[contains(., :message)]', [
+        ':message' => $message,
+      ]);
+    }
+    elseif ($aria_label) {
+      $selector = $this->buildXPathQuery($selector . '//div[@aria-label=:aria_label]', [
+        ':aria_label' => $aria_label,
+      ]);
+    }
+
+    return $selector;
+  }
+
 }