Targeting the iPhone 4 Retina Display With CSS3 Media Queries
www.iwalt.com/weblog/2010/06/targeting-the-iphone-4-retina-display-with-css3-media-queries
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Walt Dickinson has more on targeting CSS specifically for double-pixel-density devices.