@flowrider if your Mac is suddenly lost, or destroyed, now, and if the only immediately available acceptable replacement Mac runs El Capitan: you'll upgrade on day one.
I'm on a Skylake rMB and so far 4k 30FPS and 1080p60 both run flawlessly, no choppiness windowed or fullscreen.
I have a question, how do you feel about creating a browser extension for Safari so Youtube always plays in the highest available format?
Boy, thanks for clearing that up. You made up good hypothetical, why?
one-port USB-C retina MacBook Pro
As I typically do, I will upgrade my casual use Macbook Air right away, and I will hold off on upgrading my Mac Mini until the .1 or .2 release.
Wow, you bought the one-port USB-C retina MacBook Pro? Lucky. I wish I had one too. Apart from the single port issue, it's such a beast of a machine. And its modern GPU has hardware decoding of all common video sizes and framerates. I am extremely jealous. (And to answer your question, it would unfortunately be way too much work to make YouTube auto-select the highest quality. Their player does a bunch of tests to automatically pick a reasonable quality based on bandwidth, screensize, player size, browser capabilities etc.)