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

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

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

<script>
const holderElement = document.getElementById("holder");
customElements.define('my-element', class MyElement extends HTMLElement {
  constructor() {
    super();
    const shadowRoot = this.attachShadow({mode: 'open'});
    shadowRoot.innerHTML = '<style>:host { color: red; }</style>';
  }
});

PerfTestRunner.measureRunsPerSecond({
    description: "Measures performance of styling custom elements with shadowRoot's innerHTML.",

    run: function() {
      holderElement.textContent = '';
      for (i = 0; i < 100; i++) {
        holderElement.appendChild(document.createElement('my-element'));
        // force layout.
        document.body.offsetWidth;
      }
    },
    warmUpCount: 1,
});
</script>