diff --git a/html/cross-origin-embedder-policy/anonymous-iframe/broadcast-channel.tentative.window.js b/html/cross-origin-embedder-policy/anonymous-iframe/broadcast-channel.tentative.window.js
index 255a7192099232..ad64ddf5f1f568 100644
--- a/html/cross-origin-embedder-policy/anonymous-iframe/broadcast-channel.tentative.window.js
+++ b/html/cross-origin-embedder-policy/anonymous-iframe/broadcast-channel.tentative.window.js
@@ -1,3 +1,4 @@
+// META: timeout=long
 // META: script=/common/get-host-info.sub.js
 // META: script=/common/utils.js
 // META: script=/common/dispatcher/dispatcher.js
@@ -16,7 +17,15 @@ const emit_script = (key, message) => `
   bc.postMessage("${message}");
 `;
 
-promise_test(async test => {
+// For incorrect web browser implementations, some tests are failing by timing
+// out. Calling this function turns timeout into errors. This informs developers
+// about which precise test is failing.
+const errorOnTimeout = test =>
+  test.step_timeout(test.unreached_func("Timeout"), 9000);
+
+promise_test_parallel(async test => {
+  errorOnTimeout(test);
+
   const origin = get_host_info().HTTPS_REMOTE_ORIGIN;
   const key_1 = token();
   const key_2 = token();
@@ -53,4 +62,78 @@ promise_test(async test => {
   send(iframe_normal, emit_script(key_2, "msg_4"));
   assert_equals(await receive(queue_1), "msg_3");
   assert_equals(await receive(queue_2), "msg_4");
-})
+}, "Anonymous iframe and normal iframe aren't in the same partition")
+
+promise_test_parallel(async test => {
+  errorOnTimeout(test);
+
+  const origin = get_host_info().HTTPS_REMOTE_ORIGIN;
+  const key = token();
+
+  const iframe_anonymous_1 = newAnonymousIframe(origin);
+  const iframe_anonymous_2 = newAnonymousIframe(origin);
+  const queue = token();
+
+  send(iframe_anonymous_1 , listen_script(key, queue, queue));
+  assert_equals(await receive(queue), "registered");
+  send(iframe_anonymous_2, emit_script(key, "msg"));
+  assert_equals(await receive(queue), "msg");
+}, "Two sibling same-origin anonymous iframes are in the same partition");
+
+promise_test_parallel(async test => {
+  errorOnTimeout(test);
+
+  const origin = get_host_info().HTTPS_REMOTE_ORIGIN;
+  const key = token();
+  const queue = token();
+
+  const iframe_anonymous_1 = newAnonymousIframe(origin);
+  send(iframe_anonymous_1, `
+    const importScript = ${importScript};
+    await importScript("/common/utils.js");
+    await importScript("/html/cross-origin-embedder-policy/credentialless" +
+                       "/resources/common.js");
+    const newAnonymousIframe = ${newAnonymousIframe};
+    const iframe_anonymous_2 = newAnonymousIframe("${origin}");
+    send("${queue}", iframe_anonymous_2);
+  `);
+  const iframe_anonymous_2 = await receive(queue);
+
+  send(iframe_anonymous_1 , listen_script(key, queue, queue));
+  assert_equals(await receive(queue), "registered");
+  send(iframe_anonymous_2, emit_script(key, "msg"));
+  assert_equals(await receive(queue), "msg");
+}, "Nested same-origin anonymous iframe are in the same partition");
+
+promise_test_parallel(async test => {
+  errorOnTimeout(test);
+
+  const origin = get_host_info().HTTPS_REMOTE_ORIGIN;
+  const key = token();
+  const queue = token();
+
+  const iframe_anonymous_1 = newAnonymousIframe(origin);
+  const popup = newPopup(origin);
+  send(popup, `
+    const importScript = ${importScript};
+    await importScript("/common/utils.js");
+    await importScript("/html/cross-origin-embedder-policy/credentialless" +
+                       "/resources/common.js");
+    const newAnonymousIframe = ${newAnonymousIframe};
+    send("${queue}", newAnonymousIframe("${origin}"));
+  `);
+  const iframe_anonymous_2 = await receive(queue);
+
+  const unexpected_queue = token();
+  receive(unexpected_queue).then(test.unreached_func(
+    "Two same-origin anonymous iframe in different windows shouldn't be able " +
+    "to communicate using BroadcastChannel"));
+
+  send(iframe_anonymous_1 , listen_script(key, queue, unexpected_queue));
+  assert_equals(await receive(queue), "registered");
+  await send(iframe_anonymous_2, emit_script(key, "msg"));
+
+  // Wait a bit to give the opportunity for unexpected message to be received.
+  await new Promise(r => test.step_timeout(r, 500));
+}, "Two anonymous iframes in different windows do not share the same " +
+   "partition");
diff --git a/html/cross-origin-embedder-policy/anonymous-iframe/resources/common.js b/html/cross-origin-embedder-policy/anonymous-iframe/resources/common.js
index e11e2307defd6e..4fd12a6a747414 100644
--- a/html/cross-origin-embedder-policy/anonymous-iframe/resources/common.js
+++ b/html/cross-origin-embedder-policy/anonymous-iframe/resources/common.js
@@ -19,3 +19,20 @@ const newIframe = (child_origin) => {
   document.body.appendChild(iframe);
   return sub_document_token;
 };
+
+// Create a popup. The new document will execute any scripts sent toward the
+// token it returns.
+const newPopup = (origin) => {
+  const popup_token = token();
+  const popup = window.open(origin + executor_path + `&uuid=${popup_token}`);
+  return popup_token;
+};
+
+const importScript = (url) => {
+  const script = document.createElement("script");
+  script.type = "text/javascript";
+  script.src = url;
+  const loaded = new Promise(resolve => script.onload = resolve);
+  document.body.appendChild(script);
+  return loaded;
+}