<!DOCTYPE html>
<title>CSS Values and Units Test: sibling-index() changing font-weight 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 {
      font-weight: calc(100 * sibling-index());
    }
    to {
      font-weight: 600;
    }
  }
  #target {
    animation: --anim 1000s step-end;
  }
</style>
<div