<my-ce></my-ce>// [!code focus]
<script>
  class MyCe extends HTMLElement {
    constructor() {
      super()
      this.textContent = 'Hello, World!'
    }
  }
  customElements.define('my-ce', MyCe)
</script>// [!code focus]

Done 🎉

And now you can use your custom element with the tag <my-ce></my-ce>.