<!DOCTYPE html>
<head>
  <title>Iframes with loading='lazy' load when in the viewport</title>
  <link rel="author" title="Scott Little" href="mailto:sclittle@chromium.org">
  <link rel="help" href="https://github.com/scott-little/lazyload">
  <script src="/resources/testharness.js"></script>
  <script src="/resources/testharnessreport.js"></script>
</head>

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

<script>
  const t = async_test("Test that iframes with loading='lazy' load once they enter the viewport.");

  let has_window_loaded = false;
  let has_in_viewport_loaded = false;

  const in_viewport_iframe_onload = t.step_func(function() {
    assert_false(has_in_viewport_loaded, "The in_viewport element should load only once.");
    has_in_viewport_loaded = true;
  });

  window.addEventListener("load", t.step_func(function() {
    assert_true(has_in_viewport_loaded, "The in_viewport element should have loaded before window.load().");
    assert_false(has_window_loaded, "The window.load() event should only fire once.");
    has_window_loaded = true;
    document.getElementById("below_viewport").scrollIntoView();
  }));

  const below_viewport_iframe_onload = t.step_func_done(function() {
    assert_true(has_window_loaded, "The window.load() event should have fired before below_viewport loaded.");
  });
</script>

<body>
  <iframe id="in_viewport" src="resources/subframe.html?first" loading="lazy" width="200px" height="100px" onload="in_viewport_iframe_onload();">
  </iframe>
  <div style="height:10000px;"></div>
  <iframe id="below_viewport" src="resources/subframe.html?second" loading="lazy" width="200px" height="100px" onload="below_viewport_iframe_onload();">
  </iframe>

  <!--
    This async script loads very slowly in order to ensure that, if the
    below_viewport element has started loading, it has a chance to finish
    loading before window.load() happens, so that the test will dependably fail
    in that case instead of potentially passing depending on how long different
    resource fetches take.
  -->
  <script async src="/common/slow.py"></script>
</body>