[whatwg/fullscreen] How :fullscreen CSS pseudo class works in shadow dom? (#149)

After reading the [spec](https://fullscreen.spec.whatwg.org/#:fullscreen-pseudo-class), I'm still not sure how CSS pseudo-class `:fullscreen` works in the context of shadow dom. See example below:

```html
<style>
  :fullscreen { color: blue }
</style>
<my-web-component>
  #shadow-root
    <style>
      :fullscreen { color: green }
    </style>
    <video></video>
</my-web-component>
```

What happens after `await video.requestFullscreen()`?
- Is `<video>` element color green?
- Is `<my-web-component>` element color blue?

Note that I wasn't able to find web platform tests for this.


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