One of the big tradeoffs in web development is on reducing the total size of your JS files to maintain page load times low. My reasonable guess is that most websites include a similar subset of JS files. Think about jQuery, YUI and other widely used libraries. Is there a reason for browsers not to have common libraries stored locally and save an unnecessary request and download time?
closed as not constructive by grawity, Renan, 8088, Karan, Brad Patton May 13 '13 at 2:22
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Because they have no way of knowing that