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()
*/

you need to register your element
document.registerElement('smart-button', SmartButton);

https://www.polymer-project.org/1.0/articles/es6.html

and you will not be able to call your class with new
you must create it in DOM way:
document.createElement('smart-button');

or

insert in html

instead constructor you can use createdCallback()

http://h3manth.com/new/blog/2015/custom-elements-with-es6/

class CustomElement extends HTMLElement {
    // Fires when an instance of the element is created.
    createdCallback() {

    };
    // Fires when an instance was inserted into the document.
    attachedCallback() {

    };
    // Fires when an instance was removed from the document.
    detachedCallback() {

    };
    // Fires when an attribute was added, removed, or updated.
    attributeChangedCallback(attr, oldVal, newVal) {

    };
}

document.registerElement('custom', CustomElement);

let element = document.createElement('custom');

document.body.appendChild(element);