[webcomponents] Is there any plan of "custom CSS selector/property/value" spec for webcomponents? (#40)

As the title, something simply thought same as custom elements but focus on CSS.
for example, people can encapsulate and more easily use the element styles just like:

```css
apple-watch::case {
  material: steel;
}
```

Thx

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/40