STR: 1) Set Mac display sleep time to a small number of minutes. 2) Opt in to HTML5 on youtube 3) Start playing a long HTML5-enabled youtube vdeo, e.g. http://www.youtube.com/watch?v=LRWpuJRrTn4 4) Click the full screen button in youtube UI 5) Don't touch any input device 6) Wait for a larger number of minutes than the number of minutes after which the display is set to sleep Actual result: The display goes to sleep Expected results: Expected to be able to watch the long vdeo in full screen without the display going to sleep.

I'll assume you mean fullscreen video should block the screensaver, rather than fullscreen in general...