SMIL: Markup for Multimedia

The Web's HyperText Markup Language has a lousy sense of timing. In fact, HTML has no sense of time at all–unless you resort to migraine-inducing chunks of JavaScript PRE, you can't create pages in which elements appear or disappear at specific times. Too bad, because time is a cornerstone of multimedia. Look at video and multimedia authoring programs: they're built around timeline-based windows that show a project in successive stages.

http://www.macworld.com/2000/02/create/markupmultimedia.html