:picture-in-picture

:picture-in-picture {
  /* ... */
}

<h1>MDN Web Docs Demo: :picture-in-picture pseudo-class</h1>

<p>
  This demo uses the <code>:picture-in-picture</code> pseudo-class to
  automatically change the style of a video entirely using CSS.
</p>

<video id="pip-video"></video>

:picture-in-picture {
  box-shadow: 0 0 0 5px red;
}