chromium/src/third_party/blink/perf_tests/dom/custom-element-default-style.html

<script src="../resources/runner.js"></script>

<div id="holder"></div>

<script>
const holderElement = document.getElementById("holder");

const constructedStyleSheet = new CSSStyleSheet();
constructedStyleSheet.insertRule("* { color: red; }");
window.customElements.define("element-constructed", class extends HTMLElement {
  constructor() {
    super();
  }
}, { style: constructedStyleSheet });

PerfTestRunner.measureRunsPerSecond({
    description: "Measures performance of styling custom elements with default style.",

    run: function() {
      holderElement.textContent = '';
      for (i = 0; i < 100; i++) {
        holderElement.appendChild(document.createElement('element-constructed'));
        // force layout.
        document.body.offsetWidth;
      }
    },

    warmUpCount: 1,
});
</script>