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 20,045,328 websites using Device Pixel Ratio which includes location information, hosting data, contact details, 2,000,001 currently live websites and an additional 3,210,233 domains that redirect to sites in this list. 18,045,327 sites that used this technology previously and 1,072,956 websites in the United States currently using Device Pixel Ratio.