Re: [w3c/webcomponents] A new attribute similar to is="", but would allow multiple behaviors to be attached to a given element. (#662)

@prushforth 

> So are you proposing a) `<web-map mixin="map mymapbehaviour">` or b) `<map mixin="map mymapbehaviour">` where behaviour named "map" is the native behaviour that I was getting from inheriting from HTMLMapElement ? Use tr / my-tr if you have to run with that in your explanation.

I hadn't fully answered your question there.

With these element behaviors, you'd do this:

```html
<table>
  <tr has="selectable"><td></td></tr>
</table>
```

where the `<tr` is the native element, and `selectable` is a behavior applied to the `tr` and which is defined in the application space.

If we tried to achieve this with Custom Elements, we'd try:

```html
<table>
  <selectable-tr><td></td></selectable-tr>
</table>
```

which doesn't work for the reasons in #590.

`has=""` is nice because we can apply any number of **non-native** functionalities to the element:


```html
  <tr has="selectable click-logger mouse-proximity-action" onproximity="..."><td></td></tr>
```

where `tr` is still just a native builtin `tr` element, and `selectable`,  `click-logger`, and `mouse-proximity-action` are **not** native builtins, they are just simple behaviors defined in the application space. At some point, all three of those behaviors **had** to be defined in the application, with classes that _do not_ extend from `HTMLElement`:

```js
elementBehaviors.define('selectable', class { ... })
elementBehaviors.define('click-logger', class { ... })
elementBehaviors.define('mouse-proximity-action', class { ... })
```

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