packages/polyfills-loader/polyfills-loader.js
function needsTemplatePolyfill() {
  // no real <template> because no `content` property (IE and older browsers)
  const template = document.createElement('template');
  if (!('content' in template)) {
    return true;
  }
  // broken doc fragment (older Edge)
  if (!(template.content.cloneNode() instanceof DocumentFragment)) {
    return true;
  }
  // broken <template> cloning (Edge up to at least version 17)
  const template2 = document.createElement('template');
  template2.content.appendChild(document.createElement('div'));
  template.content.appendChild(template2);
  const clone = template.cloneNode(true);
  return (
    // @ts-ignore
    clone.content.childNodes.length === 0 ||
    // @ts-ignore
    clone.content.firstChild.content.childNodes.length === 0
  );
}
function needsTemplatePolyfill() {
  // no real <template> because no `content` property (IE and older browsers)
  const template = document.createElement('template');
  if (!('content' in template)) {
    return true;
  }
  // broken doc fragment (older Edge)
  if (!(template.content.cloneNode() instanceof DocumentFragment)) {
    return true;
  }
  // broken <template> cloning (Edge up to at least version 17)
  const template2 = document.createElement('template');
  template2.content.appendChild(document.createElement('div'));
  template.content.appendChild(template2);
  const clone = template.cloneNode(true);
  return (
    // @ts-ignore
    clone.content.childNodes.length === 0 ||
    // @ts-ignore
    clone.content.firstChild.content.childNodes.length === 0
  );
}
Web component polyfills loader, based on: https://github.com/webcomponents/webcomponentsjs/blob/master/webcomponents-loader.js

Adapted to not load language polyfills and use dynamic imports