YouTube currently hosts "several thousand" 3D videos to encode and play WebM formats, according to a blog post by Mozilla Director of Platform Product Management Chris Blizzard. Only people who have vision Nvidia 3D cards will be able to see videos in 3D, and only if they play in Firefox.
If you have the hardware 3D Vision, search YouTube for "yt3d" encoded videos showing 3D. You also need to configure the hardware 3D mode.
Full list of 3D HTML5 video on YouTube requirements:
• NVIDIA GeForce GPU-equipped PC or laptop
• driver NVIDIA GeForce Release 275 or more
• A 3D View Monitor Ready, laptop, projector or DLP HDTV
• NVIDIA 3D Vision emitter and glasses when using a PC monitor or laptop screen
Since the conditions mentioned above indicate, 3D HTML5 video on YouTube can only be seen by a small segment of the population surf the Web. However, the mere fact that the HTML5 video standards are still growing and the introduction of new skills, such as 3D announced today, indicates the potential that the HTML5 video is far from fully exploited. Given the continuous development and support for HTML 5 in the top 5 browsers, probably will not be long for Chrome, Internet Explorer, Safari and Opera to follow the example of Firefox here.