Synchronized Multimedia Integration Language - SMIL, pronounced "smile" 

An XML-based language that allows authors to write interactive multimedia presentations. 

An author can:
 Describe the temporal behavior of a multimedia presentation 
 Associate hyperlinks with media objects 
 Describe the layout of the presentation on a screen
 
It will allow reuse of syntax and semantics in other XML-based languages

Of particular usefulness for those who need to represent timing and synchronization.
 
Components are used for integrating timing into XHTML and into SVG. 

http://www.w3.org/TR/2001/PR-smil20-20010605/introduction.html
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CategoryInterface