How can I adjust brightness and speed while watching a video on YouTube?
If there isn't a way to do this with YouTube's Flash Player, can I do this with HTML5 video (through their beta)?
Youtube does not offer brightness/speed controls. It might be possible for a website programmer to implement such controls with HTML5, but youtube has not done so. I would recommend adjusting brightness on your monitor, or downloading the video and playing it back with a local media player program with such controls.