How to prevent my screen from dimming when watching YouTube?

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http://askubuntu.com – My screen brightness used to dim after a few seconds to preserve battery. This is default in Ubuntu 12.04. However when watching video it should not dim. This works correctly when I watch videos using native applications like VLC. With in-browser video, however, the screen is not prevented from dimming. This is very annoying as you have to move your cursor every 10 seconds or so. I used to use Mac OSX where I had the same dimming settings and Flash videos were taken into account correctly. Anyone an idea how you can make YouTube prevent your screen from dimming? (HowTos)