Also, if a user has already opted in to additional YouTube experiments, the HTML5 player may not be provided, and will default to the Flash player. If a user is using the Firefox or Opera web browsers, videos must include WebM transcodes in order to play using HTML5.
Currently HTML5 videos only provide limited full screen support–the video fills the browser, but will only truly be full screen if the browser itself supports full screen. YouTube’s HTML5 video is still a work in progress, and as such, there are still some limitations regarding its use.
If a user wishes to see a video using the WebM format, they will first have to install the WebM for IE9 codec. By default, once opted in, they will see supported videos in the h.264 format. IE9 enables viewers to watch videos using the WebM format, as well as the h.264 codec. Some browsers, such as Microsoft’s Internet Explorer 9, support more than one HTML5 video format. Once a user has opted in, they are able to use the HTML5 player rather than the traditional Adobe Flash player on supported videos (currently videos with ads are not supported, and will revert to playing in the Flash player).
YouTube is already making the change, and allows users to opt-in to its HTML5 video trial. There has been much discussion about HTML5 recently, and how it will supplant proprietary technologies such as Adobe’s Flash.