src/Tests/BigPipeNoJsDetectionTest.php             |  1 +
 .../big_pipe_test/src/BigPipeTestController.php    | 23 ++++++++++++++++++++--
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/src/Tests/BigPipeNoJsDetectionTest.php b/src/Tests/BigPipeNoJsDetectionTest.php
index 4deb9a2..75366d0 100644
--- a/src/Tests/BigPipeNoJsDetectionTest.php
+++ b/src/Tests/BigPipeNoJsDetectionTest.php
@@ -121,6 +121,7 @@ class BigPipeNoJsDetectionTest extends WebTestBase {
 
     // Keys: BigPipe no-JS placeholder markup. Values: expected replacement markup.
     $expected_big_pipe_nojs_placeholders = [
+      'big_pipe_nojs_placeholder_attribute_safe:&lt;hello' => '<marquee>Yarhar llamas forever!</marquee>',
       'big_pipe_nojs_placeholder_attribute_safe:form_action_cc611e1d' => '<form class="big-pipe-test-form" data-drupal-selector="big-pipe-test-form" action="' . base_path() . 'big_pipe_test"',
       'big_pipe_nojs_placeholder_attribute_safe:e88b559cce72c80b687d56b0e2a3a5ae4b66bc0e' => $this->container->get('csrf_token')->get('admin/appearance/default'),
     ];
diff --git a/tests/modules/big_pipe_test/src/BigPipeTestController.php b/tests/modules/big_pipe_test/src/BigPipeTestController.php
index 3b7701c..9149483 100644
--- a/tests/modules/big_pipe_test/src/BigPipeTestController.php
+++ b/tests/modules/big_pipe_test/src/BigPipeTestController.php
@@ -58,10 +58,17 @@ class BigPipeTestController extends ControllerBase {
 
     // 4. Edge case: custom string to be considered as a placeholder that
     // happens to not be valid HTML.
-    // @todo
+    $build['edge_case__invalid_html'] = [
+      '#markup' => Markup::create('<hello'),
+      '#attached' => [
+        'placeholders' => [
+          '<hello' => ['#lazy_builder' => [__CLASS__ . '::helloOrYarhar', []]],
+        ]
+      ]
+    ];
 
     // 5. Edge case: non-#lazy_builder placeholder.
-    $build['html_non_lazy_builder'] = [
+    $build['edge_case__html_non_lazy_builder'] = [
       '#markup' => Markup::create('<time>CURRENT TIME</time>'),
       '#attached' => [
         'placeholders' => [
@@ -91,4 +98,16 @@ class BigPipeTestController extends ControllerBase {
     ];
   }
 
+  /**
+   * #lazy_builder callback; says "hello" or "yarhar".
+   *
+   * @return array
+   */
+  public static function helloOrYarhar() {
+    return [
+      '#markup' => Markup::create('<marquee>Yarhar llamas forever!</marquee>'),
+      '#cache' => ['max-age' => 0],
+    ];
+  }
+
 }