Very, very few web sites support Retina-grade images as of 2013: standard images are pixel-doubled and look blurred on a Retina MacBook Pro.
Apple’s own web sites have finally begun to support the retina display in mid-2013, easily seen by browsing the 'store' pages and similar. See for example, the graphics ont the main Apple web page, e.g. the iPhone in the iOS 7 blurb front and center.
Retina grade images
Click the image below to see just how big a Retina display can be in native mode; all these pixels are drawn onto the high-density screen for a lifelike rendition in which no pixels can be seen—extreme clarity, like a 4X5 'chrome'.