Watch youtube moview on a Mac using QuickTime Is it possible to watch youtube videos on a Mac using QuickTime as it is on iOS? 
How to force Safari to render youtube video stream in QT plugin.
There’re a couple of Safari extensions that will do this. ClickToPlugin and ClickToFlash.

http://hoyois.github.com/safariextensions/clicktoplugin/

Both versions will swap out the Youtube Player for a standard HTML5 player. The extension also allows you to download the Youtube video.