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HTML5 is a natural evolution of HTML 4 that accounts for the rapid growth of media, rich online experiences, and sophisticated Web application development since HTML 4.01 became a specification at the end of 1999.
At the time of this writing, HTML5 is still under development and subject to change. However, it is on stable footing, and browsers have already added many—and continue to add more—of its features. (Please see http://www.htmlfiver.com/html5-browser-support/ for more information.)