Re: [webcomponents] [Custom Elements] the `extends` property cannot be replaced (#326)

I've been  told to CC @annevk , @rniwa , @hayatoito , and @travisleithead about this, so here i am.

The long discussion about `is` property and the `extends` one is that "anyone could wrap an input, a select, a form ... " etc, etc ...

When it comes to partial applications, the element cannot be just inherited, and `is` behavior, as well as `extends` is the only way to resolve the issue.

### Example
In a partial application we might want to create a `CustomElement` that should act lake a `HTMLTableRowElement`

```js
var MyTr = document.registerElement('my-tr', {prototype: Object.create(HTMLTableRowElement.prototype)});
var mtr = new MyTr;
mtr.innerHTML = '<td>1</td><td>2</td>';
mtr.innerHTML // "12" <== SEE THAT? EPIC FAIL
```

If the component just inherits `HTMLTableRowElement.prototype` it can be used to be appended to a generic `tbody` but it won't preserve its content as html.

Only using the good old way works:
```js
var mtr = document.createElement('tr');
mtr.innerHTML = '<td>1</td><td>2</td>';
mtr.innerHTML // "<td>1</td><td>2</td>" <== OK
```

Partial rows,  cells, cols or any other table related application can be very useful.

As example, [this replication of the (in)famous DBMonster benchmark](https://github.com/WebReflection/dom-class/blob/master/demo/db-monster.js) uses 3 components:

  * the `HTMLTableElement` one to create a table component
  * the `HTMLTableRowElement` one to create [each row](https://github.com/WebReflection/dom-class/blob/master/demo/db-monster.js#L86) of such table
  * the `HTMLTableCellElement` to create [last 5 cells](https://github.com/WebReflection/dom-class/blob/master/demo/db-monster.js#L33) of each rows

The [benchmark page](https://github.com/WebReflection/dom-class/blob/master/demo/dbmonster.html) as it is [works like a charm](http://webreflection.github.io/dom-class/demo/dbmonster.html) in every [polyfilled](https://github.com/WebReflection/document-register-element) platform but it would be impossible to recreate such simple use case (even if stressed in this bench) without the usage of the `is` specification.


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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/326#issuecomment-142115167