Updating the bug type. Please undo the changes if its not the case.

I was wondering if there is a timeline for this or if perhaps the progress of this issue is being tracked elsewhere?

Thanks for your interest!
You can star this issue to be updated when development will start.

Out of curiosity, do you have a use case in mind already?

 I have a web based HUD dashboard showing information in real time. The information is mostly basic (albeit important) and can be show in a relatively small area. I have an Android table mounted to the wall running a native app showing other info, too. It would be great if I could float a small window with the web info on top of everything else! 

(On another note, being able to pip any html element (not just a video) would be nice, too, but I can work around that for now by copying the element to canvas and canvas to video)

FYI, if your Android tablet is on Android O, you can play fullscreen a "canvas" video and hit the "Home" button so that video goes in Picture-in-Picture automatically (without the Picture-in-Picture Web API).

Thanks. It's not on O, it's the original Nexus 7 (old!). 
The canvas is not in the Dom, so I don't think the non API method would work. While I could change the above, it seems the web API is the more intuitive approach.