YouTube has started to make it possible to playback 4K video with a framerate of 60 frames per second. Previously videos with a 4K / Ultra HD resolution and 60 FPS could already be uploaded to the video platform, but there weren't playable yet.
To playback the videos fluently and in native resolution, suitable hardware is required, like a monitor that can display a 3840x2160 pixel resolution. Also a fast internet connection is required.
The number of 4K / 60FPS videos is still very limited, the playlist uploaded by Youtube developers currently exists of 6 videos. It appears Google's video site currently mainly wants to experiment with the video resolution, e.g. to determine which bitrates work best.
In 2010 Google already introduced the possibility to upload 4K videos, but these had a maximum frame rate of 25 or 30 frames per second. Since December 2013 those videos could also be playbacked through the player of the video platform.
