<!DOCTYPE html>
<style>
.x > div, .x > span { background-color: yellow; }
</style>
<script src="util.js"></script>
<script>
window.onload = function() {
  let root = document.createElement("div");
  root.appendChild(document.createElement("div"));
  document.body.appendChild(root);
  flush_layout(root);
  perf_start();
  root.firstChild.className = "x";
  flush_style(root);
  perf_finish();
};
</script>
<body></body>
