YouTube: Flash Is Dead, Long Live Flash
Along with HTML 5, open standards such as Cascading Style Sheets (CSS 3), Scalable Vector Graphics (SVG) and Web Open Font Format (WOFF) for typography can imitate the features of Flash. The issue is one of portability--Flash is already used to stream video to multiple browsers, on multiple platforms and devices.
YouTube programmer John Harding stated the reasons why HTML 5 hasn't taken over on the video site, stating "While HTML5's video support enables us to bring most of the content and features of YouTube to computers and other devices that don't support Flash Player, it does not yet meet all of our needs. Today, Adobe Flash provides the best platform for YouTube's video distribution requirements, which is why our primary video player is built with it."