wpt/css/css-values/tree-counting/sibling-index-keyframe-value-dynamic.html at master · web-platform-tests/wpt · GitHub

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<!DOCTYPE html>
<title>CSS Values and Units Test: sibling-index() changing value during @keyframes animation</title>
<link rel="help" href="https://drafts.csswg.org/css-values-5/#tree-counting">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  @keyframes --anim {
    from {
      z-index: sibling-index();
    }
    to {
      z-index: 1;
    }
  }
  #target {
    animation: --anim 1000s step-end;
    position: relative;
    width: 100px;
    height: 100px;
    background: red;
  }
  #abs {
    position: absolute;
    width: 100px;
    height: 100px;
    z-index: 3;
    background: green;
  }
</style>
<p>You should see a green square below.</p>
<div>
  <div id="rm"></div>
  <div id="abs"></div>
  <div id="target"></div>
</div>
<script>
  test(() => {
    assert_equals(getComputedStyle(target).zIndex, "3");
  }, "Initially, the sibling-index() is 3 for #target");

  test(() => {
    rm.remove();
    assert_equals(getComputedStyle(target).zIndex, "2");
  }, "Removing a preceding sibling of #target reduces the sibling-index()");

</script>