Re: [WICG/webcomponents] "open-stylable" Shadow Roots (#909)

@bahrus I don't think `<slot>` makes much sense outside of shadow DOM because you end up with contention of the children. Composition works because the light and shadow DOM are separated.

Say a component rendering to it's own light DOM, something like:

```html
<my-card>
  <slot name="title">Alert</slot>
  <slot></slot>
  <button>OK</button>
</my-card>
```

And then a user would like to use it:

```html
<my-element>
  <h1 slot="title">Welcome</h1>
  <p>Thanks for reading my card...</p>
</my-element>
```

What should render? When? We have two timings to deal with: 1) the usual content exists before element's 2) the element's content exist before the users. In 1) The element could accidentally overwrite the user's content. Or it could append to itself. Then it would have to move the user's content into the slot, but the user wouldn't know that and could keep appending into what's now the element's semi-private DOM. In 2) If the user uses a template system like vdom, they may remove all the element's content.

The only way to get this to work in any stable way is make a contract that separates user-provided children from element provided with a special child element. Then either all user provided content must go in the child, or all element provided:

And then you can't really use `<slot>` because that would be projecting content from an outer scope. So i'll invent `<content>` and `<content-slot>` to allow projection between siblings:

```html
<my-element>
  <content>
    <h1 slot="title">Welcome</h1>
    <p>Thanks for reading my card...</p>
  </content>
  <content-slot name="title">Alert</content-slot>
  <content-slot></content-slot>
  <button>OK</button>
</my-element>
```

This gets really complicated really quick. There's a reason I keep rejecting this feature in LitElement.

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