Safari also supported. Be sure to select at least 720p.
YouTube is huge, and gaming on YouTube is not only massive, but increasing in size and stature all the time. Recently, the online video platform announced that 60fps (as well as 48fps) would be rolled out in full. Naturally, many types of videos (GoPro, cough) will benefit from higher framerate support, but gaming, which consumes much of the internet's bandwidth as it is, in particular looks to be a prime way to exhibit 60fps support.
While previously available in certain example videos, 60fps support is now spreading to the full community. As part of this initial roll-out, viewing 60fps YouTube videos is restricted to Google Chrome and Safari. Support for other browsers is expected soon.
Here's a great early example:
(Be sure to watch in HD.)