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text/plain when you link to their "raw" versions, like this. GitHub does this on purpose as to not encourage hotlinking of those files, which is a bad idea in general anyhow.
On a site like CodePen, which is great for little demos and tests that aren't usually mission-critical, it is often useful to hotlink styles and scripts like that. So this is just a heads up that that won't work anymore in Chrome soon. Chrome is on a six week cycle, so this change will go out to stable Chrome around May 16, 2013. You could always test in another browser, or probably better, move that script somewhere else to link to. You can host the file right here on CodePen if you're PRO.