firefox/testing/web-platform/tests/speculation-rules/inline-speculation-rules-errors.html

<!DOCTYPE html>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script>
setup({ allow_uncaught_exception: true });

promise_test(async t => {
  const script = document.createElement('script');
  script.type = 'speculationrules';
  script.textContent = 'invalid json';

  const log = [];
  const elementError = new Promise(resolve => {
    script.addEventListener('error', e => {
      assert_equals(e.constructor, Event, 'event should be a simple Event');
      log.push('element error');
      resolve();
    }, { once: true });
  });

  const globalError = new Promise(resolve => {
    window.addEventListener('error', e => {
      e.preventDefault();
      assert_equals(e.constructor, ErrorEvent, 'global event should be an ErrorEvent');
      assert_equals(e.error.constructor, TypeError, 'e.error should be a TypeError');
      log.push('global error');
      resolve();
    }, { once: true });
  });

  document.head.appendChild(script);

  await Promise.all([elementError, globalError]);
  assert_array_equals(log, ['element error', 'global error']);
}, 'A script with invalid JSON should fire error events on the element and window');
</script>