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chromium/third_party/blink/web_tests/external/wpt/css/css-anchor-position/anchor-position-top-layer-ref.html 

<!DOCTYPE html>
<title>Tests anchor positioning with top-layer elements</title>

<style>
body {
  margin: 0;
  height: 300vh;
}

#anchor {
  position: fixed;
  top: 200px;
  left: 200px;
  width: 100px;
  height: 100px;
  background: orange;
}

#target {
  position: fixed;
  top: 200px;
  left: 300px;
  width: 100px;
  height: 100px;
  background: lime;
}
</style>

<div id="anchor"></div>
<div id="target"></div>

<script>
document.scrollingElement.scrollTop = 100;
</script>