mozilla-release/testing/web-platform/tests/resource-timing/status-codes-create-entry.html

<!doctype html>
<html>
<head>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
</head>
<body>
<img src="resources/status-code.py?status=200">
<img src="resources/status-code.py?status=307">
<img src="resources/status-code.py?status=404">
<img src="resources/status-code.py?status=502">
<script>
async_test(t => {
  window.addEventListener("load", t.step_func(() => {
    const images = document.getElementsByTagName("img");
    for (let img of images) {
      assert_greater_than(performance.getEntriesByName(img.src).length, 0, img.src);
    }
    t.done();
  }));
}, "Make sure all status codes are reported");
</script>