Given the specification is still in DRAFT with only one browser engine -- webkit -- supporting it, I would NEVER suggest anyone use this on a deployment site.
Deploying draft specifications on live sites is what made us spend a decade and a half fighting "quirks mode". It's what led to the derpy painful "browser prefix" trash. It is NEVER a good idea.
Much less you suggest a polyfill... is there even such a thing? If there is I'd like to see it.
It's fun to play with though, to see what we might be able to use real-world in another decade... well, unless like grid it magically overnight gains support from all current engines. Something that right now there doesn't seem to be much traction on.
NOT that I ever expect anything new to show up in Safari at this point. Webkit has been aging like milk ever since crazy Goog absconded with all the talented dev's.