First you will need “Click to Plugin”. This turns off instant loading of Flash,WMV,PDF,and any other kind of video/audio plugin you can think of. You will want to immediately go to the preferences and enable certain ones to always pass through (like Google Maps for instance).

Second, you will need “Media Center” to easily control playback to the AppleTV. This extension adds a toolbar icon that you can add by going to (Safari) View/Customize Toolbar… You will see a new icon to drag down that reads “Airplay” in the shape of a square. Too bad it’s not the familiar Airplay icon we’re used to, but no matter. It works!

Then when you go to a YouTube page that movie will be loaded via HTML5. Pressing the “F” key will play it fullscreen. A floating menu gives you the option to stream it to Airplay, download the file, change the source and more options. It’s really fantastic.

For more information consult the author’s page and this blog post from MacStories.