How to use Flash® Player for YouTube™
By default Flash® Player for YouTube™ forces YouTube™ to play videos using Flash® Player. Each time that a video is loaded the message "Your browser does not currently recognize any of the video formats available" (or another similar message) is displayed during one or two seconds then the default HTML5 player is automatically replaced by the Flash® Player. If the message lasts more than a few seconds the page must be reloaded.
You can quickly disable or enable Flash® Player for YouTube™ to use either the default HTML5 player or the Flash® Player by clicking the button that has been added to your browser. Please note that when you click it you need to reload the video(s) already loaded if it's not done automatically, otherwise you may not see the changes.