:picture-in-picture

The :picture-in-picture pseudo-class lets you configure your stylesheets to automatically adjust the size, style, or layout of content when a video switches back and forth between picture-in-picture and traditional presentation modes.

In this example, a video has a box shadow when it is displayed in the floating window.

The page's HTML looks like this:

<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>

The <video> with the ID "pip-video" will change between having a red box shadow or not, depending on whether or not it is displayed in the picture-in-picture floating window.

The magic happens in the CSS.

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

• Picture-in-picture API
• HTMLVideoElement.requestPictureInPicture()
• HTMLVideoElement.autoPictureInPicture
• HTMLVideoElement.disablePictureInPicture
• Document.pictureInPictureEnabled
• Document.exitPictureInPicture()
• Document.pictureInPictureElement