macOS Mojave offers a convenient — and fun — Picture in Picture (PiP) feature. You can “float” a video window from Safari or iTunes over your desktop or a full-screen app.
You can play the video in any corner of the desktop and resize it to see more or less of what’s behind it. The video even stays put when you switch Spaces.
There are some catches, however. When it comes to Safari, Web developers have to use the PiP application programming interface (API) that Apple provides to make Picture in Picture is available. If a site doesn’t support PiP, you won’t be able to use it.
To use PiP in Safari, launch the web browser and cruise on over to any site that supports the feature. Begin playing a video. Click on the PiP icon on the video control bar at the lower right hand side of the screen. A small window will appear in a corner of your screen playing the video.
To use PiP in iTunes, launch the app and begin playing a video. Click on the PiP icon on the video control bar at the lower right hand side of the screen. Again, a small window will appear in a corner of your screen playing the video.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)