chromium/src/third_party/blink/web_tests/http/tests/priority-hints/image-low-importance-in-viewport.html

<script src=../priorities/resources/common.js></script>
<script src=../resources/testharness.js></script>
<script src=../resources/testharnessreport.js></script>

<script>
  const t = async_test('low importance on in-viewport images must be fetched with kLow resource load priority');
  const url = new URL('../resources/slow-image.php?sleep=1000&name=square.png', location);
  const priorityPromise = internals.getResourcePriority(url, document);

  // Synchronous wait to delay layout by .5 seconds
  const end = Date.now() + 500;
  while (Date.now() < end) {}

  priorityPromise.then(priority => {
    assert_equals(priority, kLow);
    t.done();
  });
</script>

<!--
  In-viewport images get re-prioritized if layout is computed mid-flight. With importance=low, re-prioritization should have no effect,
  we delay both the image request (will be fetched by preload scanner) by a second, and finish layout mid-flight. As long as the final
  resource load priority is kLow, we know the image was not re-prioritized.
-->
<img importance=low src="../resources/slow-image.php?sleep=1000&name=square.png" alt=img>