Slots

<my-ce>// [!code focus]
  <span>Hello, Slots!</span>// [!code focus]
</my-ce>// [!code focus]
<script>
  class MyCe extends HTMLElement {
    constructor() {
      super()
      const shadow = this.attachShadow({ mode: 'open' })
      shadow.innerHTML = '<slot></slot>'
    }
  }
  customElements.define('my-ce', MyCe)
</script>// [!code focus]