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-prefix-free let you use only unprefixed CSS properties everywhere. It works behin the scene, adding the current browser’s prefix to any CSS code, only when it’s needed.

my advice : It would be a great tool but it’s only compatible with IE9+, Opera 10+, Firefox 3.5+, Safari 4+ and Chrome. The lack of backward compatibility is really pain in the ass and you may consider using other tools like less or saas.