Source Code: (back to article)
<div id="outer"></div>

<script>
const outer = document.getElementById('outer');
const shadow = outer.attachShadow({ mode: 'open' });
const button = document.createElement('button');
button.textContent = 'Click me';

button.addEventListener('click', event => {
const composedInfo = document.createElement('p');
composedInfo.textContent = `Composed: ${event.composed}`;
shadow.appendChild(composedInfo);
});

shadow.appendChild(button);
</script>