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Because you're not using IE 8? Or not something that supports CSS 3 besides Chrome/Safari, Firefox, and Opera?
It's outputting the things that it tries, and output for all but the last means the browser does not support it. Because if the browser did support it then the loop/function would end thanks to that return statement. My Chrome only outputs WebkitTransition because it supports that; Firefox would output what you saw, Opera would output the three, and basically every other browser would output all four.