I use the ClickToPlugin extension. It stops plug ins, mainly flash, from loading automatically and when possible it exchanges the player to safaris native html5 video player. Random flash adds on website dont load in either. Click the gray screen saying html5, or flash or whatever else to let it load in. For youtube it will change the youtube player automatically because its in included in the html5 converstion skripts list in the settings of click to plugin. Make sure you add "www.youtube.com/watch" (without " marks) into the autoplay these sites in the media player tab of the clicktoplugin settings so that you dont have to click the video every time.
A nice side effect of using this plugin is that youtube adds no longer load. Just goes straight to the video

html5 also uses a million times less CPU power, so you can watch videos for much longer.
Oh and I personally like the html5 player much more then youtubes, you can jump around in a video however you like and it wont have to reload

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The quality settings (resolution) from each video can be changed by hovering your mouse over the lop left corner of the screen. This cant be done when you are in fullscreen though.
You can also send the video to your apple tv if you have one.