mozilla-release/testing/web-platform/tests/loading/lazyload/original-base-url-applied-tentative.html

<!DOCTYPE html>
<head>
  <title>Deferred iframes and images with loading='lazy' use the original base URL specified at the parse time</title>
  <link rel="author" title="Raj T" href="mailto:rajendrant@chromium.org">
  <script src="/resources/testharness.js"></script>
  <script src="/resources/testharnessreport.js"></script>
  <script src="common.js"></script>
</head>

<!--
Marked as tentative until https://github.com/whatwg/html/pull/3752 is landed.
-->

<script>
  const below_viewport_iframe = new ElementLoadPromise("below_viewport_iframe");
  const below_viewport_img = new ElementLoadPromise("below_viewport_img");

  // Change the base URL and scroll down to load the deferred elements.
  window.addEventListener("load", () => {
    window.history.pushState(1, document.title, '/invalid-url-where-no-subresources-exist/')
    below_viewport_iframe.element().scrollIntoView();
  });

  async_test(function(t) {
    below_viewport_iframe.promise.then(
      t.step_func_done(function() {
        assert_true(below_viewport_iframe.element().contentDocument.body.innerHTML.includes("<p>Subframe</p>"));
      }));
  }, "Test that when deferred iframe is loaded, it uses the base URL computed at parse time.");

  async_test(function(t) {
    below_viewport_img.promise.then(
      t.step_func_done(function() {
        assert_true(below_viewport_img.element().complete);
        assert_greater_than(below_viewport_img.element().naturalWidth, 0);
      })
    ).catch(t.unreached_func("The image load should not fail, trying to load with invalid base URL."));
  }, "Test that when deferred img is loaded, it uses the base URL computed at parse time.");
</script>

<body>
  <div style="height:10000px;"></div>
  <script>
    // Change the base URL so that the iframe makes use of that in its relative
    // URL to absolute URL computation at parse time.
    window.history.pushState(1, document.title, 'resources/')
  </script>
  <iframe id="below_viewport_iframe" src="subframe.html" loading="lazy" width="200px" height="100px" onload="below_viewport_iframe.resolve();">
  </iframe>
  <img id="below_viewport_img" src="image.png" loading="lazy" onload="below_viewport_img.resolve();"
       onerror="below_viewport_img.reject();">
</body>