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! And this solution works on Mac as well as iPad!
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 or up (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: