Google Chrome now enables picture-in-picture as a default setting on Mac, Windows, and Linux computers as of Chrome 70. If you are up-to-date and watching a compatible video in the Chrome web browser, you can minimize it and continue browsing the web in other tabs, while the video keeps playing in a new miniature screen (via Android Police).
The feature works similarly to Safari's implementation of PIP: on compatible websites you can two-finger click twice on a playing video to find "Picture in Picture." This will pause the video on the main tab, turn it black, and display the video in a new window that can be moved around anywhere on the screen.
PIP was previously in the Chrome 69 beta but it had to be manually enabled, so it appears that Google is making it easier for users to gain access to the feature with Chrome 70. PIP still isn't available on every video-playing website since it will have to be adopted by each site, but you can enable PIP with YouTube in Chrome on macOS starting today.