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diff --git a/tests/wpt/tests/css/css-values/tree-counting/sibling-index-keyframe-rotate-dynamic.html b/tests/wpt/tests/css/css-values/tree-counting/sibling-index-keyframe-rotate-dynamic.html
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+++ b/tests/wpt/tests/css/css-values/tree-counting/sibling-index-keyframe-rotate-dynamic.html
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+<!DOCTYPE html>
+<title>CSS Values and Units Test: sibling-index() changing rotate 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 {
+      rotate: x calc(10deg * sibling-index());
+    }
+    to {
+      rotate: x 90deg;
+    }
+  }
+  #target {
+    animation: --anim 1000s step-end;
+  }
+</style>
+<div>
+  <div id="rm"></div>
+  <div></div>
+  <div id="target"></div>
+</div>
+<script>
+  test(() => {
+    assert_equals(getComputedStyle(target).rotate, "x 30deg");
+  }, "Initially, the sibling-index() is 3 for #target");
+
+  test(() => {
+    rm.remove();
+    assert_equals(getComputedStyle(target).rotate, "x 20deg");
+  }, "Removing a preceding sibling of #target reduces the sibling-index()");
+
+</script>