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

<!DOCTYPE html>
<title>CSS Anchor Positioning: Transition when @position-try is applied</title>
<link rel="help" href="https://drafts.csswg.org/css-anchor-position-1/#fallback-apply">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  #cb {
    display: inline-block;
    position: relative;
    width: 400px;
    height: 250px;
    border: 1px solid black;
  }
  #cb.shrink {
    width: 300px;
  }
  #target {
    position: absolute;
    left: 300px;
    width: 50px;
    height: 50px;
    background: skyblue;
    position-try-fallbacks: --200;
  }
  #target.anim {
    transition: left 1000s steps(2, start);
  }
  @position-try --200 {
    left: 200px;
  }
</style>
<div id=cb>
  <div id=target></div>
</div>
<script>
  function cleanup() {
    target.className = '';
    cb.className = '';
  }

  test((t) => {
    t.add_cleanup(cleanup);
    assert_equals(target.offsetLeft, 300);
    cb.classList.add('shrink');
    target.classList.add('anim');
    // Now we take the --200 fallback:
    assert_equals(target.offsetLeft, 250);
  }, 'Transition when @position-try is applied');

  test((t) => {
    t.add_cleanup(cleanup);
    cb.classList.add('shrink');
    assert_equals(target.offsetLeft, 200);
    cb.classList.remove('shrink');
    target.classList.add('anim');
    assert_equals(target.offsetLeft, 250);
  }, 'Transition when @position-try is unapplied');
</script>