Introduction

There currently exists a Web API for putting an HTMLVideoElement into a
Picture-in-Picture window (HTMLVideoElement.requestPictureInPicture()). This
limits a website's ability to provide a custom picture-in-picture experience
(PiP). We want to expand upon that functionality by giving websites the ability
to open a picture-in-picture (i.e., always-on-top) window with a blank document
that can be populated with arbitrary HTMLElements instead of only a single
HTMLVideoElement.

This new window will be much like a blank same-origin window opened via the
existing window.open() API, with some differences:

• The PiP window will float on top of other windows.
• The PiP window will never outlive the opening window.
• The website cannot set the position of the PiP window.
• The PiP window cannot be navigated (any window.history or window.location
  calls that change to a new document will close the PiP window).
• The PiP window cannot open more windows.

Read the complete Explainer.

Feedback

I welcome feedback in this thread, but encourage you to file bugs against the Explainer.