chromium/third_party/blink/perf_tests/css/CSSCountersCreate.html

<!DOCTYPE html>
<script src="../resources/runner.js"></script>
<style>
  div {
    counter-increment: my-counter;
  }
  div::before {
    content: counter(my-counter);
  }
</style>
<body></body>
<script>
  function createTree(root, num_elements, depth) {
    if (!depth) {
      return;
    }
    for (let i = 0; i < num_elements; ++i) {
      const div = document.createElement('div');
      root.append(div);
      createTree(div, num_elements, depth - 1);
    }
  }

  PerfTestRunner.measureTime({
    description: 'Measure the time of the css counters creation',
    run: function() {
      document.body.innerHTML = '';
      createTree(document.body, 5, 6);
    },
    iterationCount: 5,
  });
</script>