diff --git a/browsertest-convert.php b/browsertest-convert.php
index a75f8b9..cc20c34 100644
--- a/browsertest-convert.php
+++ b/browsertest-convert.php
@@ -539,6 +539,10 @@ function glob_recursive($pattern, $flags = 0) {
       && strpos($simpletest_file_path, 'UrlAliasFixtures.php') === FALSE
       && strpos($simpletest_file_path, 'FieldDefaultValueCallbackProvider.php') === FALSE
       && strpos($simpletest_file_path, 'FakeRecord.php') === FALSE
+      && strpos($simpletest_file_path, 'StubForm.php') === FALSE
+      && strpos($simpletest_file_path, 'TestDataConverter.php') === FALSE
+      && strpos($simpletest_file_path, 'ErrorContainer.php') === FALSE
+      && strpos($simpletest_file_path, 'ExceptionContainer.php') === FALSE
     ) {
       unlink($simpletest_file_path);
     }
diff --git a/core/modules/system/tests/modules/test_page_test/src/Controller/Test.php b/core/modules/system/tests/modules/test_page_test/src/Controller/Test.php
index 35ccf0c..7bb0fb1 100644
--- a/core/modules/system/tests/modules/test_page_test/src/Controller/Test.php
+++ b/core/modules/system/tests/modules/test_page_test/src/Controller/Test.php
@@ -88,4 +88,14 @@ public function error() {
     ];
   }
 
+  /**
+   * Renders a page with encoded markup.
+   *
+   * @return array
+   *   A render array as expected by drupal_render()
+   */
+  public function renderEncodedMarkup() {
+    return ['#plain_text' => 'Bad html <script>alert(123);</script>'];
+  }
+
 }
diff --git a/core/modules/system/tests/modules/test_page_test/test_page_test.routing.yml b/core/modules/system/tests/modules/test_page_test/test_page_test.routing.yml
index a2d5ed5..cbcf6d8 100644
--- a/core/modules/system/tests/modules/test_page_test/test_page_test.routing.yml
+++ b/core/modules/system/tests/modules/test_page_test/test_page_test.routing.yml
@@ -58,3 +58,11 @@ test_page_test.error:
     code: 200
   requirements:
     _access: 'TRUE'
+
+test_page_test.encoded:
+  path: '/test-encoded'
+  defaults:
+    _title: 'Page with encoded HTML'
+    _controller: '\Drupal\test_page_test\Controller\Test::renderEncodedMarkup'
+  requirements:
+    _access: 'TRUE'