fune/testing/web-platform/tests/webmessaging/messageerror.html

<!DOCTYPE html>
<title>onmessageerror content attribute</title>
<link rel="author" title="Domenic Denicola" href="mailto:d@domenic.me">
<link rel="help" href="https://html.spec.whatwg.org/multipage/#handler-onmessageerror">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

<body>

<script>
"use strict";
window.messageErrorHappened = false;

function cleanup() {
  window.messageErrorHappened = false;
  document.body.removeAttribute("onmessageerror");
}

test(() => {
  assert_equals(document.body.onmessageerror, null, "body");
  assert_equals(window.onmessageerror, null, "window");
}, "The default value of onmessageerror is null");

test(t => {
  t.add_cleanup(cleanup);

  document.body.setAttribute("onmessageerror", "window.messageErrorHappened = true;")
  const compiledHandler = document.body.onmessageerror;

  assert_equals(typeof compiledHandler, "function", "The onmessageerror property must be a function");
  compiledHandler();
  assert_true(window.messageErrorHappened, "Calling the handler must run the code");
}, "The onmessageerror content attribute must be compiled into the onmessageerror property");

test(t => {
  t.add_cleanup(cleanup);

  document.body.setAttribute("onmessageerror", "window.messageErrorHappened = true;")

  window.dispatchEvent(new Event("messageerror"));

  assert_true(window.messageErrorHappened, "Dispatching the event must run the code");
}, "The onmessageerror content attribute must execute when an event is dispatched on the window");
</script>