imsky/holder — pull request #31 by ksonnad (2013-01-27 11:21:03+00:00)
Title: Check and account for high-res canvas.

Retina MacBook Pros running Safari 6 automatically increase the pixel
ratio of canvas elements, so also multiplying by the devicePixelRatio
is excessive.

http://www.html5rocks.com/en/tutorials/canvas/hidpi/