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chromium/third_party/blink/web_tests/external/wpt/css/css-scroll-anchoring/anchor-inside-iframe.html 

<!DOCTYPE html>
<link rel="help" href="https://drafts.csswg.org/css-scroll-anchoring-1/">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<iframe width="700" height="500" srcdoc="
  <!DOCTYPE html>
  <style> body { height: 1000px } div { height: 100px } </style>
  <div id='block1'>abc</div>
  <div id='block2'>def</div>
"></iframe>
<script>
  async_test((t) => {
    var iframeWindow = document.querySelector("iframe").contentWindow;
    iframeWindow.addEventListener("load", () => {
      var block1 = iframeWindow.document.querySelector("#block1");
      iframeWindow.scrollTo(0, 150);

      requestAnimationFrame(() => {
        step_timeout(() => {
          block1.style.height = "200px";
          assert_equals(iframeWindow.scrollY, 250);
          t.done();
        }, 0);
      });
    });
  }, "Scroll anchoring in an iframe.");
</script>