[w3c/webcomponents] Having slots as element prevents styling empty containers (#701)

Having slots as element prevents styling empty containers, e.g.

```html
<my-layout-component>
  <slot name="foo"></slot>
</my-layout-component>
```

Say I need to hide `my-layout-component` if empty, this won't work:

```css
my-layout-component:empty {
  display: none;
}
```

because there's the `slot`, even if I do not place any element in it via `slot="foo"`.

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