The desire to move way from Flash and other plugins for Web content is quite apparent, and has taken a further step with last week's release of Firefox 3.6. In response to the progress in HTML5 development, sites like YouTube and Vimeo have released HTML5 versions of their media implementations. Despite this, the instructions for enabling the beta implementation of HTML5 on YouTube are not the clearest.
By the descriptions of the "new player", people may assume there is a separate site, or a separate player on the website that you can view side-by-side with the older Flash player; however, this is not the case. YouTube uses a cookie to store the settings for which player to load, whenever the main YouTube page is loaded.
To change the settings in this cookie, go to the HTML5 beta player page (https://www.youtube.com/html5) and click the "Join the HTML5 Beta" link. When you do this, the link will change to "Leave the HTML5 Beta", and you're done! At this point, just go back to the YouTube main page and content that is supported in HTML5 will be displayed using HTML5 instead of Flash.
At this point, you can check by playing video on YouTube and checking "Activity Monitor" to see if Flash Player is running and using the CPU. Without the HTML5 player you should only see Flash using a good amount of CPU, but when you are using the HTML5 player you should only see Safari (or whatever browser you use) using more CPU.
Overall you will not see much difference between the HTML5 and Flash players, and not all movies will be available in HTML5. There will also be a few limitations to start out with, including not being able to run at full screen for some movies, but regardless of these limitations, the HTML5 player may be more attractive for those who are experiencing problems and frustrations with Flash.