How do I change the brightness and contrast of YouTube videos on-the-fly?

I don't believe there is a way to change the brightness and contrast of YouTube videos while they are playing. Flash Player does not control the video brightness or contrast of video settings. This would first be determined when the video is produced, and after that through monitor/screen settings.

My only ideas are :

• Uninstall and reinstall the Adobe Flash player, in the hope that something went wrong when it was updating itself.
• Try another browser than the one you are using, to see if the Flash player implementation for your browser is broken on your computer.
• Search for an update to your display driver on the manufacturer's website
• Download the Youtube video and use another player than Youtube's

To adjust the monitor's brightness, DimScreen might be a freeware solution :

• Change the brightness by selecting a `% in the tray menu.
• Or use the hotkeys Ctrl++ and Ctrl+-.
• Change hotkeys using Settings in the tray menu.
• Doesn't work properly with video windows.

I don't know if this is what you are looking for but Powerstrip maps hotkeys to change brightness. It's not free though.

Not being able to do so directly in YouTube, I stream them through VLC player, and use the brightness control in that player.

Copy the YouTube video URL and paste into the VLC player's Media -> Open Network Stream network tab. As the video streams through VLC, you'll have all the graphics features associated with the player.