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"Dropping support for IE8 means we can take advantage of the best parts of CSS without being held back with CSS hacks or fallbacks".

This sure contributed to reducing the size.

We had so few IE8 specific hacks. I don't think it really made a large impact honestly.

Similarly, jQuery 2 shrunk 12% because of cutting out IE≤8 hacks: http://blog.jquery.com/2013/04/18/jquery-2-0-released/