.../EventSubscriber/DefaultExceptionSubscriber.php | 71 +---------------------
 .../Comment/CommentResourceTestBase.php            |  2 +-
 .../DefaultExceptionSubscriberTest.php             |  1 -
 3 files changed, 4 insertions(+), 70 deletions(-)

diff --git a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
index 7b5ecc1..fdc2385 100644
--- a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
@@ -7,7 +7,6 @@
 use Drupal\Core\StringTranslation\StringTranslationTrait;
 use Drupal\Core\Utility\Error;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
-use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
 use Symfony\Component\HttpKernel\Exception\HttpExceptionInterface;
@@ -59,12 +58,12 @@ protected function getErrorLevel() {
   }
 
   /**
-   * Handles any exception as a generic error page for HTML.
+   * Handles exceptions for this subscriber.
    *
    * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
    *   The event to process.
    */
-  protected function onHtml(GetResponseForExceptionEvent $event) {
+  public function onException(GetResponseForExceptionEvent $event) {
     $exception = $event->getException();
     $error = Error::decodeException($exception);
 
@@ -132,71 +131,7 @@ protected function onHtml(GetResponseForExceptionEvent $event) {
   }
 
   /**
-   * Handles an HttpExceptionInterface exception for unknown formats.
-   *
-   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
-   *   The event to process.
-   */
-  protected function onFormatUnknown(GetResponseForExceptionEvent $event) {
-    /** @var \Symfony\Component\HttpKernel\Exception\HttpExceptionInterface|\Exception $exception */
-    $exception = $event->getException();
-
-    $response = new Response($exception->getMessage(), $exception->getStatusCode(), $exception->getHeaders());
-    $event->setResponse($response);
-  }
-
-  /**
-   * Handles errors for this subscriber.
-   *
-   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
-   *   The event to process.
-   */
-  public function onException(GetResponseForExceptionEvent $event) {
-    $format = $this->getFormat($event->getRequest());
-    $exception = $event->getException();
-
-    $method = 'on' . $format;
-    if (!method_exists($this, $method)) {
-      if ($exception instanceof HttpExceptionInterface) {
-        $this->onFormatUnknown($event);
-        $response = $event->getResponse();
-        $response->headers->set('Content-Type', 'text/plain');
-      }
-      else {
-        $this->onHtml($event);
-      }
-    }
-    else {
-      $this->$method($event);
-    }
-  }
-
-  /**
-   * Gets the error-relevant format from the request.
-   *
-   * @param \Symfony\Component\HttpFoundation\Request $request
-   *   The request object.
-   *
-   * @return string
-   *   The format as which to treat the exception.
-   */
-  protected function getFormat(Request $request) {
-    $format = $request->query->get(MainContentViewSubscriber::WRAPPER_FORMAT, $request->getRequestFormat());
-
-    // These are all JSON errors for our purposes. Any special handling for
-    // them can/should happen in earlier listeners if desired.
-    if (in_array($format, ['drupal_modal', 'drupal_dialog', 'drupal_ajax'])) {
-      $format = 'json';
-    }
-
-    return $format;
-  }
-
-  /**
-   * Registers the methods in this class that should be listeners.
-   *
-   * @return array
-   *   An array of event listener definitions.
+   * {@inheritdoc}
    */
   public static function getSubscribedEvents() {
     $events[KernelEvents::EXCEPTION][] = ['onException', -256];
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php
index b04da54..223a1ee 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Comment/CommentResourceTestBase.php
@@ -281,7 +281,7 @@ public function testPostDxWithoutCriticalBaseFields() {
     // @todo Uncomment, remove next 3 lines in https://www.drupal.org/node/2820364.
     $this->assertSame(500, $response->getStatusCode());
     $this->assertSame(['text/plain; charset=UTF-8'], $response->getHeader('Content-Type'));
-    $this->assertSame('Internal Server Error', (string) $response->getBody());
+    $this->assertStringStartsWith('The website encountered an unexpected error. Please try again later.</br></br><em class="placeholder">Symfony\Component\HttpKernel\Exception\HttpException</em>: Internal Server Error in <em class="placeholder">Drupal\rest\Plugin\rest\resource\EntityResource-&gt;post()</em>', (string) $response->getBody());
     //$this->assertResourceErrorResponse(422, "Unprocessable Entity: validation failed.\nentity_type: This value should not be null.\n", $response);
 
     // DX: 422 when missing 'entity_id' field.
diff --git a/core/tests/Drupal/Tests/Core/EventSubscriber/DefaultExceptionSubscriberTest.php b/core/tests/Drupal/Tests/Core/EventSubscriber/DefaultExceptionSubscriberTest.php
index 6582500..279d830 100644
--- a/core/tests/Drupal/Tests/Core/EventSubscriber/DefaultExceptionSubscriberTest.php
+++ b/core/tests/Drupal/Tests/Core/EventSubscriber/DefaultExceptionSubscriberTest.php
@@ -18,7 +18,6 @@ class DefaultExceptionSubscriberTest extends UnitTestCase {
 
   /**
    * @covers ::onException
-   * @covers ::onFormatUnknown
    */
   public function testOnExceptionWithUnknownFormat() {
     $config_factory = $this->getConfigFactoryStub();