A: Usually, youtube.com uses Flash for its video display. However, it does have an HTML5 version that also uses either the h.264 or WebM format.
Go to YouTube's site for HTML5, and it will check your browser's capabilities. It will also let you join the HTML5 trial.
In the screen shot below, you can see that my browser supports all the HTML5 requirements. This is good if you're using the Metro Internet Explorer in Windows 8 previews that doesn't allow plug-ins, such as Flash.
To see more FAQs, please go to John Savill's FAQs page on Windows IT Pro.