In Safari 5, HTML5 video can deliver an even richer, more accessible experience. Watch video embedded with the HTML5 video tag in full screen. If the video includes closed captions, simply click the closed-captions button in the controls to display them.Captioning is particularly interesting; unfortunately, I've searched around and have been unable to find a coherent explanation of how you actually make a caption work with video.
Apple: Safari Technology Demos HTML 5 Showcase: Video
Apple: Explanation of Demo: Video Effects