chromium/src/third_party/blink/web_tests/external/wpt/shadow-dom/declarative/support/helpers.js

function setInnerHTML(el,content) {
  const fragment = (new DOMParser()).parseFromString(`<pre>${content}</pre>`, 'text/html', {includeShadowRoots: true});
  (el instanceof HTMLTemplateElement ? el.content : el).replaceChildren(...fragment.body.firstChild.childNodes);
}