Notes on context.webkitBackingStorePixelRatio
Basically the intended use for this new API is the following.
You have your canvas as is, forget the world of retina displays, just think of CSS units, not physical pixels. Physical pixels have now been abstracted away.
You create your canvas in your document, you set your canvas to be 940px wide since your website is on a 940 pixel grid (for example).
You begin drawing to your canvas, and everything just works. You get high resolution lines, high resolution circles, shapes, etc. The canvas, while you interact with it in a lower resolution, is automagically upscaled internally for you, as if you where drawing vectors.