YouTube is experimenting with a shorter, more compatible embed code which uses the < iframe > tag.
If you choose to use the new embed code style, site visitors will be able to view the embedded video in one of YouTube's Flash or HTML5 players depending on their viewing environment and preferences. Supported environments include those browsers that support both the video tag in HTML5 and either the h.264 video codec or the WebM format (with VP8 codec).
As an aside, WebM is a media file format designed exclusively for the Web.
The suppported browsers include Firefox (WebM enabled version), Google Chrome (h.264 supported), Opera (WebM supported), Microsoft IE 9 (h.264), and Apple Safari (h.264, version 4+). Should there be an instance when HTML5 is not supported, YouTube will use Flash.
One of the most significant developments of the new embed style is that it will eventually allow embeds to work on mobile devices.
There are a few restrictions to the beta-test of the new embed code. Videos with ads are not supported (they will play in the Flash player), and on Firefox and Opera only videos with WebM transcodes will play in HTML5.
To participate in this experiment, simply join the HTML5 beta here.