All you need to do is simply go to www.youtube.com/html5
and Click the button where it says

Join the HTML5 Beta

Make sure you use the supported browser to use HTML5
Supported Browsers for HTML5 (data based on 4/19/2010)

Supported Browsers

Right now we support browsers that support both the video tag in HTML5 and the h.264 video codec. These include:
  •    Google Chrome
  •    Apple Safari (version 4+)
  •   Microsoft Internet Explorer with Google Chrome Frame installed (Get Google Chrome Frame)


simply.. think of it as an replacement for Adobe Flash Player.
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