Extend DOM elements ES6

es6-dom-extend.js

class SmartButton extends HTMLButtonElement {
  constructor() {}
}
let sb = new SmartButton();
document.body.appendChild(sb);

/*
I get the below error:

TypeError: Failed to execute 'appendChild' 
on 'Node': parameter 1 is not of type 'Node'.

It needs a document.registerElement as well? :/

But then what's the whole point of extending, it can just do a Object.create()
*/