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chromium/third_party/blink/web_tests/external/wpt/css/css-anchor-position/anchor-transition-default.html 

<!DOCTYPE html>
<title>CSS Anchor Positioning: Transition when position-anchor changes</title>
<link rel="help" href="https://drafts4.csswg.org/css-anchor-position-1/">
<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/9598">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  #cb {
    display: inline-block;
    position: relative;
    width: 250px;
    height: 250px;
    border: 1px solid black;
  }
  #anchor1, #anchor2 {
    width: 100px;
  }
  #anchor1 {
    background: wheat;
    height: 50px;
    anchor-name: --a1;
  }
  #anchor2 {
    background: tomato;
    height: 90px;
    anchor-name: --a2;
  }
  #anchored {
    position: absolute;
    width: anchor-size(width);
    height: anchor-size(height);
    top: anchor(top);
    left: anchor(right);
    transition-duration: 1000s;
    transition-timing-function: steps(2, start);
    transition-property: top, height;
    background-color: skyblue;
    position-anchor: --a1;
  }
</style>
<div id=cb>
  <div id=anchor1>Anchor1</div>
  <div id=anchor2>Anchor2</div>
  <div id=anchored></div>
</div>
<script>
  test(() => {
    assert_equals(anchored.offsetTop, 0);
    assert_equals(anchored.offsetHeight, 50);
    anchored.style.positionAnchor = '--a2';
    assert_equals(anchored.offsetTop, 25);
    assert_equals(anchored.offsetHeight, 70);
  }, 'Transition when position-anchor changes');
</script>