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chromium/third_party/blink/web_tests/external/wpt/html/semantics/embedded-content/the-img-element/image-loading-lazy-available.html 

<!doctype html>
<title>The list of available images gets checked before deciding to make a load lazy</title>
<link rel="help" href="https://html.spec.whatwg.org/multipage/images.html#update-the-image-data">
<link rel="help" href="https://html.spec.whatwg.org/multipage/images.html#will-lazy-load-image-steps">
<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1709577">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

<img src="/images/green-256x256.png">
<div style="height:1000vh;"></div>
<script>
promise_test(async t => {
  await new Promise(resolve => {
    window.addEventListener("load", resolve);
  });
  let nonLazy = document.querySelector("img");
  assert_equals(nonLazy.width, 256);
  assert_equals(nonLazy.height, 256);

  let lazy = document.createElement("img");
  lazy.loading = "lazy";
  lazy.src = nonLazy.src;
  document.body.appendChild(lazy);

  await new Promise(resolve => setTimeout(resolve));

  assert_equals(lazy.width, 256, "The list of available images should be checked before delaying the image load");
  assert_equals(lazy.height, 256, "The list of available images should be checked before delaying the image load");
});
</script>