Quinn from Snazzy Labs just posted an interesting video about some macOS utilities. I love small utilities, but I like it even better, when a problem can be solved with system functions. That is the case here. He mentions Helium - an app to use a form of picture in picture mode for websites that don’t support the native PIP.
However, there is a better solution!
You can use a small bookmarklet that pushes the current HTML5 video on screen into the native PIP. And yes, it also works with Netflix, if you are on macOS Mojave (as far as I can tell, Netflix doesn’t run in the previous versions of Safari).
Create a new bookmark, enter the following string as the address, give it a nice emoji (like this: ⤵️), voilà! You can now use the native PIP for nearly all your videos! Here is the code: