Adds a CSS pseudo-class to help web developers customize the media player when videos enter and exit Picture-in-Picture.
Docs: https://wicg.github.io/picture-in-picture/#css-pseudo-class
Samples: https://googlechrome.github.io/samples/picture-in-picture/css-pseudo-class
Explainers: https://github.com/WICG/picture-in-picture/pull/120