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chromium/third_party/blink/web_tests/external/wpt/css/css-anchor-position/anchor-animation-dynamic-name.html 

<!DOCTYPE html>
<title>Tests CSS animation of anchor(), dynamic anchor-name changes</title>
<link rel="help" href="https://drafts.csswg.org/css-anchor-position-1/">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  #cb {
    border: 1px solid black;
    width: 400px;
    height: 400px;
    position: relative;
  }
  #a1, #a2 {
    width: 100px;
    height: 100px;
  }
  #a1 {
    background: tomato;
  }
  #a2 {
    background: seagreen;
  }
  .anchor { anchor-name: --a; }
  #anchored {
    width: 50px;
    height: 50px;
    background: skyblue;
    animation: --anim 1s steps(2, start);
    position: absolute;
    position-anchor: --a;
  }
  @keyframes --anim {
    from { top: anchor(top); }
    to { top: anchor(bottom); }
  }
</style>
<div id=cb>
  <div id=a1 class=anchor></div>
  <div id=a2></div>
  <div id=anchored></div>
</div>
<script>
  test(() => {
    assert_equals(anchored.offsetTop, 50);
    a1.classList.toggle('anchor');
    a2.classList.toggle('anchor');
    assert_equals(anchored.offsetTop, 150);
  }, 'Animation with anchor() responds to anchor-name change');
</script>