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 20,563,828 websites using Device Pixel Ratio which includes location information, hosting data, contact details, 2,000,001 currently live websites and an additional 3,223,654 domains that redirect to sites in this list. 18,563,827 sites that used this technology previously and 1,077,548 websites in the United States currently using Device Pixel Ratio.