HTML5 video is the Web standard that allows users of most modern-day browsers to play hosted videos in their browser without having to rely on Adobe's Flash player.
In order to use the new feature, users will need to opt in to the HTML5 program on TestTube, which is where the company houses a number of other experimental features.
The only browsers that are currently supported include Google Chrome, Apple's Safari, and Internet Explorer with the Chrome frame installed. Not included on the list are Opera or Mozilla's Firefox, despite the fact that recent versions of both browsers work with the HTML5 video spec.
Along with the browser limitations, watching YouTube videos in HTML5 mode requires that the videos being watched are free of ads, user-created captions and annotations.
Note: As of 6:22 p.m. PST, the feature does not yet appear to be functioning site-wide. A Google rep told CNET that the company's engineers were just beginning to push it out, and that it would be live on the site in the next "hour or two."
Related: HTML groups tackle Webcam support