customizedBuiltIn.js

class CoolButton extends HTMLButtonElement {
  constructor() {
    super(); // must call as usual
  }
  
  connectedCallback() {
    this.addEventListener('click', this._onClick);
    // Woohoo, we've been connected
  }
  
  _onClick() {
    console.log('Yep, just clicked me');
  }
}

customElements.define("cool-button", CoolButton, {extends: "button"});