350/900 (rounded values)
But it drops frames when I click pause, drops during loading... It's unwatchable.
I'm not sure what's the point of 8k at this point, other than 'just for fun', or maybe just for benchmarks or advertising purposes of new devices. Ah, possibly preparing for virtual reality devices, they really need a high resolution.
8k might work on GPUs that are optimized for 8k, but even then, 8k monitors, connection speeds, HTML 5 are all an issue.
I doubt that HTML 5 rendering is well suited for 8k.
That said, the 30 frames version is completely watchable using an external player capable of YouTube playback (60 frames doesn't even work). Does it look great on an 1080p monitor? It looks sharp, but aliased. I'm sure that a quality encoded 1080p video could look better.