diff --git a/webmessaging/broadcastchannel/detached-iframe.tentative.html b/webmessaging/broadcastchannel/detached-iframe.tentative.html
new file mode 100644
index 000000000000000..b9b06c3a46463bf
--- /dev/null
+++ b/webmessaging/broadcastchannel/detached-iframe.tentative.html
@@ -0,0 +1,174 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/get-host-info.sub.js"></script>
+<!-- Pull in the with_iframe helper function from the service worker tests -->
+<script src="/service-workers/service-worker/resources/test-helpers.sub.js"></script>
+<body>
+<script>
+const TEST_IFRAME_CLASS_NAME = 'test-iframe';
+const events = [];
+var bc1;
+const DONE_MSG = 'done';
+
+function DetachedIframeTestCheckForOneMessage(t) {
+  return new Promise((resolve) => {
+    bc1.onmessage = t.step_func(e => {
+      events.push(e);
+      if (e.data == DONE_MSG) {
+        assert_equals(events.length, 1);
+        resolve();
+      }
+    });
+  });
+}
+
+const IframeAction = {
+  REMOVE_BEFORE_CREATION: 'remove-before-creation',
+  REMOVE_AFTER_CREATION: 'remove-after-creation',
+};
+
+async function doMessageSentTest(t, channelName, action) {
+  await with_iframe('about:blank');
+  const iframe = document.getElementsByClassName(TEST_IFRAME_CLASS_NAME)[0];
+  const iframe_BroadcastChannel = iframe.contentWindow.BroadcastChannel;
+
+  if (action === IframeAction.REMOVE_BEFORE_CREATION) {
+    iframe.remove();
+  }
+
+  events.length = 0;
+
+  bc1 = new BroadcastChannel(channelName);
+  const bc2 = new BroadcastChannel(channelName);
+  const iframe_bc = new iframe_BroadcastChannel(channelName);
+
+  if (action === IframeAction.REMOVE_AFTER_CREATION) {
+    iframe.remove();
+  }
+
+  const testResultsPromise = DetachedIframeTestCheckForOneMessage(t);
+
+  iframe_bc.postMessage('test');
+  bc2.postMessage(DONE_MSG);
+
+  bc2.close();
+  iframe_bc.close();
+  t.add_cleanup(() => bc1.close());
+
+  return testResultsPromise;
+}
+
+promise_test(async t => {
+  return doMessageSentTest(
+      t, 'postMessage-from-detached-iframe-pre',
+      IframeAction.REMOVE_AFTER_CREATION);
+}, 'BroadcastChannel messages from detached iframe to parent should be ignored (BC created before detaching)');
+
+promise_test(async t => {
+  return doMessageSentTest(
+      t, 'postMessage-from-detached-iframe-post',
+      IframeAction.REMOVE_BEFORE_CREATION);
+}, 'BroadcastChannel messages from detached iframe to parent should be ignored (BC created after detaching)');
+
+
+async function doMessageReceivedTest(t, channelName, action) {
+  await with_iframe('about:blank');
+  const iframe = document.getElementsByClassName(TEST_IFRAME_CLASS_NAME)[0];
+  const iframe_BroadcastChannel = iframe.contentWindow.BroadcastChannel;
+
+  if (action === IframeAction.REMOVE_BEFORE_CREATION) {
+    iframe.remove();
+  }
+
+  events.length = 0;
+
+  // `iframe_bc` must be created first so that it receives messages before
+  // `bc1`. That way we can tell whether `iframe_bc` received a message by
+  // inspecting `events` in the `bc1` message handler.
+  const iframe_bc = new iframe_BroadcastChannel(channelName);
+  iframe_bc.onmessage = e => {
+    events.push(e)
+  };
+  bc1 = new BroadcastChannel(channelName);
+  const bc2 = new BroadcastChannel(channelName);
+
+  if (action === IframeAction.REMOVE_AFTER_CREATION) {
+    iframe.remove();
+  }
+
+  const testResultsPromise = DetachedIframeTestCheckForOneMessage(t);
+  bc2.postMessage(DONE_MSG);
+
+  bc2.close();
+  iframe_bc.close();
+  t.add_cleanup(() => bc1.close());
+}
+
+promise_test(async t => {
+  return doMessageReceivedTest(
+      t, 'postMessage-to-detached-iframe-pre',
+      IframeAction.REMOVE_AFTER_CREATION);
+}, 'BroadcastChannel messages from parent to detached iframe should be ignored (BC created before detaching)');
+
+promise_test(async t => {
+  return doMessageReceivedTest(
+      t, 'postMessage-to-detached-iframe-post',
+      IframeAction.REMOVE_BEFORE_CREATION);
+}, 'BroadcastChannel messages from parent to detached iframe should be ignored (BC created after detaching)');
+
+
+async function doMessageSendReceiveTest(t, channelName, action) {
+  await with_iframe('about:blank');
+  const iframe = document.getElementsByClassName(TEST_IFRAME_CLASS_NAME)[0];
+  const iframe_BroadcastChannel = iframe.contentWindow.BroadcastChannel;
+
+  if (action === IframeAction.REMOVE_BEFORE_CREATION) {
+    iframe.remove();
+  }
+
+  const iframe_bc1 = new iframe_BroadcastChannel(channelName);
+  const iframe_bc2 = new iframe_BroadcastChannel(channelName);
+  iframe_bc1.onmessage = t.unreached_func(
+      'Detached iframe BroadcastChannel instance received message unexpectedly');
+
+  if (action === IframeAction.REMOVE_AFTER_CREATION) {
+    iframe.remove();
+  }
+
+  iframe_bc2.postMessage(DONE_MSG);
+
+  iframe_bc2.close();
+  t.add_cleanup(() => iframe_bc1.close());
+
+  // To avoid calling t.step_timeout here, instead just create two new
+  // BroadcastChannel instances and complete the test when a message is passed
+  // between them.  Per the spec, all "BroadcastChannel objects whose relevant
+  // agents are the same" must have messages delivered to them in creation
+  // order, so if we get this message then it's safe to assume the earlier
+  // message would have been delivered if it was going to be.
+  const bc1 = new BroadcastChannel(channelName);
+  const bc2 = new BroadcastChannel(channelName);
+  return new Promise((resolve) => {
+    bc1.onmessage = t.step_func(e => {
+      resolve();
+    });
+    bc2.postMessage(DONE_MSG);
+  });
+}
+
+promise_test(async t => {
+  return doMessageSendReceiveTest(
+      t, 'postMessage-within-detached-iframe-pre',
+      IframeAction.REMOVE_AFTER_CREATION);
+}, 'BroadcastChannel messages within detached iframe should be ignored (BCs created before detaching)');
+
+promise_test(async t => {
+  return doMessageSendReceiveTest(
+      t, 'postMessage-within-detached-iframe-post',
+      IframeAction.REMOVE_BEFORE_CREATION);
+}, 'BroadcastChannel messages within detached iframe should be ignored (BCs created after detaching)');
+
+</script>
+</body>
diff --git a/webmessaging/broadcastchannel/resources/service-worker.js b/webmessaging/broadcastchannel/resources/service-worker.js
index fc37aede84586e2..a3d17b9c650adb5 100644
--- a/webmessaging/broadcastchannel/resources/service-worker.js
+++ b/webmessaging/broadcastchannel/resources/service-worker.js
@@ -9,6 +9,7 @@ bc3.onmessage = e => {
 };
 bc3.postMessage('from worker');
 
+// Ensure that the worker stays alive for the duration of the test
 self.addEventListener('install', evt => {
   evt.waitUntil(promise);
 });
diff --git a/webmessaging/broadcastchannel/resources/worker.js b/webmessaging/broadcastchannel/resources/worker.js
index 9364c602756385c..df23072bc99f95d 100644
--- a/webmessaging/broadcastchannel/resources/worker.js
+++ b/webmessaging/broadcastchannel/resources/worker.js
@@ -16,6 +16,12 @@ function handler(e, reply) {
   c = new BroadcastChannel(e.data.channel);
   let messages = [];
   c.onmessage = e => {
+      if (e.data === 'ready') {
+        // Ignore any 'ready' messages from the other thread since there could
+        // be some race conditions between this BroadcastChannel instance
+        // being created / ready to receive messages and the message being sent.
+        return;
+      }
       messages.push(e.data);
       if (e.data == 'done')
         reply(messages);
diff --git a/webmessaging/broadcastchannel/workers.html b/webmessaging/broadcastchannel/workers.html
index 1d05feb7edbf0e9..16fea069309f573 100644
--- a/webmessaging/broadcastchannel/workers.html
+++ b/webmessaging/broadcastchannel/workers.html
@@ -80,11 +80,14 @@
 
     let worker = new Worker('resources/worker.js');
     worker.onmessage = t.step_func(e => {
-        assert_array_equals(events, ['c1: from worker', 'c2: echo'],
+        assert_array_equals(events,
+                            ['c1: from worker', 'c2: ready', 'c2: echo'],
                             'messages in document');
         assert_array_equals(e.data, ['done'], 'messages in worker');
         t.done();
       });
+    worker.onmessagerror =
+        t.unreached_func('Worker\'s onmessageerror handler called');
 
     c.addEventListener('message', e => {
         if (e.data == 'from worker') {
@@ -94,48 +97,30 @@
               let c2 = new BroadcastChannel('worker-close');
               c2.onmessage = e => {
                   events.push('c2: ' + e.data);
-                  c2.postMessage('done');
+                  if (e.data === 'ready') {
+                    worker.postMessage({ping: 'echo'});
+                  } else {
+                    c2.postMessage('done');
+                    c2.close();
+                  }
                 };
-              worker.postMessage({ping: 'echo'});
+              // For some implementations there may be a race condition between
+              // when the BroadcastChannel instance above is created / ready to
+              // receive messages and when the worker calls postMessage on it's
+              // BroadcastChannel instance. To avoid this, confirm that our
+              // instance can receive a message before indicating to the other
+              // thread that we are ready. For more details, see:
+              // https://github.com/whatwg/html/issues/7267
+              let c3 = new BroadcastChannel('worker-close');
+              c3.postMessage('ready');
+              c3.close();
             }, 1);
         }
       });
 
     worker.postMessage({channel: 'worker-close'});
+    t.add_cleanup(() => worker.terminate());
 
   }, 'Closing and re-opening a channel works.');
 
-async_test(t => {
-  function workerCode() {
-    close();
-    var bc = new BroadcastChannel('worker-test');
-    postMessage(true);
-  }
-
-  var workerBlob = new Blob([workerCode.toString() + ";workerCode();"], {type:"application/javascript"});
-
-  var w = new Worker(URL.createObjectURL(workerBlob));
-  w.onmessage = function(e) {
-    assert_true(e.data, "BroadcastChannel created on worker shutdown.");
-    t.done();
-  }
-}, 'BroadcastChannel created after a worker self.close()');
-
-async_test(t => {
-  function workerCode() {
-    close();
-    var bc = new BroadcastChannel('worker-test-after-close');
-    bc.postMessage(true);
-  }
-
-  var bc = new BroadcastChannel('worker-test-after-close');
-  bc.onmessage = function(e) {
-    assert_true(e.data, "BroadcastChannel created on worker shutdown.");
-    t.done();
-  }
-
-  var workerBlob = new Blob([workerCode.toString() + ";workerCode();"], {type:"application/javascript"});
-  new Worker(URL.createObjectURL(workerBlob));
-}, 'BroadcastChannel used after a worker self.close()');
-
 </script>
diff --git a/webmessaging/broadcastchannel/workers.tentative.html b/webmessaging/broadcastchannel/workers.tentative.html
new file mode 100644
index 000000000000000..e72fa365fd78613
--- /dev/null
+++ b/webmessaging/broadcastchannel/workers.tentative.html
@@ -0,0 +1,256 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+
+async_test(t => {
+  function workerCode() {
+    close();
+    try {
+      var bc = new BroadcastChannel('worker-create-after-close');
+    } catch (e) {
+      postMessage(e);
+      return;
+    }
+    postMessage(true);
+  }
+
+  var workerBlob = new Blob(
+      [workerCode.toString() + ';workerCode();'],
+      {type: 'application/javascript'});
+
+  var w = new Worker(URL.createObjectURL(workerBlob));
+  w.onmessage = t.step_func_done(function(e) {
+    assert_equals(
+        e.data, true,
+        'BroadcastChannel creation in closed worker triggered exception: ' +
+            e.data.message);
+  });
+  t.add_cleanup(() => w.terminate());
+}, 'BroadcastChannel created after a worker self.close()');
+
+
+function postMessageFromWorkerWorkerCode(workerName, channelName) {
+  if (workerName === 'close-before-create-worker') {
+    close();
+  }
+  let bc = new BroadcastChannel(channelName);
+  if (workerName === 'close-after-create-worker') {
+    close();
+  }
+  bc.postMessage(workerName + ' done');
+  postMessage(true);
+}
+
+function doPostMessageFromWorkerTest(t, workerName, channelName) {
+  var bc = new BroadcastChannel(channelName);
+  bc.onmessage = t.step_func_done(function(e) {
+    assert_equals(
+        e.data, 'done-worker done',
+        'BroadcastChannel message should only be received from the second worker');
+  });
+  t.add_cleanup(() => bc.close());
+
+  var testMessageHandler = t.step_func(function(e) {
+    assert_equals(
+        e.data, true,
+        'Worker sent postMessage indicating it sent a BroadcastChannel message');
+
+    var w = createWorker(
+        postMessageFromWorkerWorkerCode, 'done-worker', channelName);
+    t.add_cleanup(() => w.terminate());
+  });
+  createWorker(
+      postMessageFromWorkerWorkerCode, workerName, channelName,
+      testMessageHandler);
+
+  // To avoid calling t.step_timeout here, have the worker postMessage(true)
+  // once it is finished and then we'll instantiate another worker that
+  // performs the same test steps but doesn't close. By the time the
+  // BroadcastChannel message in that worker gets sent successfully it should
+  // be safe to assume that any BroadcastChannel messages from the previous
+  // worker would have been sent if they were going to be.
+}
+
+function createWorker(workerCode, workerName, channelName, handler = null) {
+  var workerCodeStr = workerCode.toString() +
+      `;${workerCode.name}("${workerName}", "${channelName}");`;
+  var workerBlob = new Blob([workerCodeStr], {type: 'application/javascript'});
+  var w = new Worker(URL.createObjectURL(workerBlob));
+  if (handler !== null) {
+    w.onmessage = handler;
+  }
+  return w;
+}
+
+async_test(t => {
+  const workerName = 'close-after-create-worker';
+  const channelName = workerName + '-postmessage-from-worker';
+  doPostMessageFromWorkerTest(t, workerName, channelName);
+}, 'BroadcastChannel messages from closed worker to parent should be ignored (BC created before closing)');
+
+async_test(t => {
+  const workerName = 'close-before-create-worker';
+  const channelName = workerName + '-postmessage-from-worker';
+  doPostMessageFromWorkerTest(t, workerName, channelName);
+}, 'BroadcastChannel messages from closed worker to parent should be ignored (BC created after closing)');
+
+
+function postMessageToWorkerWorkerCode(workerName, channelName) {
+  self.addEventListener('message', () => {
+    if (workerName === 'close-before-create-worker') {
+      close();
+    }
+    try {
+      let bc1 = new BroadcastChannel(channelName);
+      bc1.onmessage = e => {
+        if (e.data === 'ready') {
+          postMessage(e.data);
+        } else if (e.data === 'test') {
+          postMessage(workerName + ' done');
+        }
+      };
+      bc1.onmessageerror = () => {
+        postMessage('onmessageerror called from worker BroadcastChannel');
+      };
+      if (workerName === 'close-after-create-worker') {
+        close();
+      }
+    } catch (e) {
+      postMessage(e);
+      return;
+    }
+
+    if (workerName === 'done-worker') {
+      // For some implementations there may be a race condition between when
+      // the BroadcastChannel instance above is created / ready to receive
+      // messages and when the parent calls postMessage on it's
+      // BroadcastChannel instance. To avoid this, confirm that our instance
+      // can receive a message before indicating to the other thread that we
+      // are ready. For more details, see:
+      // https://github.com/whatwg/html/issues/7267
+      let bc2 = new BroadcastChannel(channelName);
+      bc2.postMessage('ready');
+      bc2.close();
+    } else {
+      // Since the worker has closed, it's not expected that the
+      // BroadcastChannel will receive messages (there's a separate test for
+      // that), so just indicate directly that it's ready to test receiving
+      // a message from the parent dispite the possibility of a race condition.
+      postMessage('ready');
+    }
+  });
+  self.addEventListener('messageerror', () => {
+    postMessage('onmessageerror called from worker');
+  });
+}
+
+function doPostMessageToWorkerTest(t, workerName, channelName) {
+  var bc = new BroadcastChannel(channelName);
+  t.add_cleanup(() => bc.close());
+
+  var doneMessageHandler = t.step_func(function(e) {
+    if (e.data === 'ready') {
+      bc.postMessage('test');
+    } else if (e.data === 'done-worker done') {
+      t.done();
+    } else {
+      assert_unreached(
+          'BroadcastChannel.postMessage triggered exception within second worker: ' +
+          e.data.message);
+    }
+  });
+  var testMessageHandler = t.step_func(function(e) {
+    assert_equals(
+        e.data, 'ready',
+        'Worker sent postMessage indicating its BroadcastChannel instance is ready');
+    bc.postMessage('test');
+
+    var doneWorker = createWorker(
+        postMessageToWorkerWorkerCode, 'done-worker', channelName,
+        doneMessageHandler);
+    t.add_cleanup(() => {
+      doneWorker.terminate();
+    });
+    doneWorker.postMessage('start');
+  });
+  var testWorker = createWorker(
+      postMessageToWorkerWorkerCode, workerName, channelName,
+      testMessageHandler);
+  testWorker.postMessage('start');
+}
+
+async_test(t => {
+  const workerName = 'close-after-create-worker';
+  const channelName = workerName + '-postmessage-to-worker';
+  doPostMessageToWorkerTest(t, workerName, channelName);
+}, 'BroadcastChannel messages from parent to closed worker should be ignored (BC created before closing)');
+
+async_test(t => {
+  const workerName = 'close-before-create-worker';
+  const channelName = workerName + '-postmessage-to-worker';
+  doPostMessageToWorkerTest(t, workerName, channelName);
+}, 'BroadcastChannel messages from parent to closed worker should be ignored (BC created after closing)');
+
+
+function postMessageWithinWorkerWorkerCode(workerName, channelName) {
+  if (workerName === 'close-before-create-worker') {
+    close();
+  }
+  try {
+    let bc1 = new BroadcastChannel(channelName);
+    let bc2 = new BroadcastChannel(channelName);
+    bc1.onmessage = e => {
+      postMessage(workerName + ' done')
+    };
+    if (workerName === 'close-after-create-worker') {
+      close();
+    }
+    bc2.postMessage(true);
+    postMessage(true);
+  } catch (e) {
+    postMessage(e);
+  }
+}
+
+function doPostMessageWithinWorkerTest(t, workerName, channelName) {
+  var doneMessageHandler = t.step_func(function(e) {
+    if (e.data === true) {
+      // Done worker has finished - no action needed
+    } else if (e.data === 'done-worker done') {
+      t.done();
+    } else {
+      assert_unreached(
+          'BroadcastChannel.postMessage triggered exception within second worker: ' +
+          e.data.message);
+    }
+  });
+  var testMessageHandler = t.step_func(function(e) {
+    assert_equals(
+        e.data, true,
+        'Worker indicated that the test procedures were executed successfully');
+
+    var w = createWorker(
+        postMessageWithinWorkerWorkerCode, 'done-worker', channelName,
+        doneMessageHandler);
+    t.add_cleanup(() => w.terminate());
+  });
+  createWorker(
+      postMessageWithinWorkerWorkerCode, workerName, channelName,
+      testMessageHandler);
+}
+
+async_test(t => {
+  const workerName = 'close-after-create-worker';
+  const channelName = workerName + '-postmessage-within-worker';
+  doPostMessageWithinWorkerTest(t, workerName, channelName);
+}, 'BroadcastChannel messages within closed worker should be ignored (BCs created before closing)');
+
+async_test(t => {
+  const workerName = 'close-before-create-worker';
+  const channelName = workerName + '-postmessage-within-worker';
+  doPostMessageWithinWorkerTest(t, workerName, channelName);
+}, 'BroadcastChannel messages within closed worker should be ignored (BCs created after closing)');
+
+</script>
