puppeteer/test/assets/shadow.html

<script>

let h1 = null;
window.button = null;
window.clicked = false;

window.addEventListener('DOMContentLoaded', () => {
  const shadowRoot = document.body.attachShadow({mode: 'open'});
  h1 = document.createElement('h1');
  h1.textContent = 'Hellow Shadow DOM v1';
  button = document.createElement('button');
  button.textContent = 'Click';
  button.addEventListener('click', () => clicked = true);
  shadowRoot.appendChild(h1);
  shadowRoot.appendChild(button);
});
</script>