A media query to display different CSS based on the device pixel ratio. Standard screens have a pixel ratio of 1, retina devices have a pixel ratio greater than 1.
http://www.hongkiat.com/blog/css-retina-display/
CSS Media Queries
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Get a list of 21,086,517 websites using Device Pixel Ratio which includes location information, hosting data, contact details, 2,000,001 currently live websites and an additional 2,988,997 domains that redirect to sites in this list. 19,086,516 sites that used this technology previously and 1,036,390 websites in the United States currently using Device Pixel Ratio.