macOS Sierra now supports video picture-in-picture just like the iPad for Safari and iTunes. You can pop out a video from the web into a floating overlay that stays on top of other windows and snaps to corners or anywhere on your display when you hold Option.
It’s a really nice feature when it’s available, but a lot of websites do not yet expose a PIP button. Luckily, Developer Arno Appenzeller has released a free Safari extension that enables PIP for Safari, Netflix and more video services. Here’s how to use it: