Related to this is https://www.w3.org/Bugs/Public/show_bug.cgi?id=15041 which is a request to clarify the dimensions of the image returned by toDataURL. 

In WebKit, the toDataURL routine returns an image corresponding in dimensions to that of the backing store (device pixels), which can be larger than that of the canvas coordinate space. In this case, the test toDataURL.png.primarycolours.htm fails because the canvas draws the image obtained from toDataURL back onto the canvas without specifying a destination width and height, which defaults to the source image width and height, which is (as explained above) not necessarily the same as the canvas width and height. 

In order to make this test work regardless of the resolution of the backing store, the destination width and height should be provided. 

We should change:

ctx.drawImage(img, 0, 0);

to

ctx.drawImage(img, 0, 0, 100, 50);

Link: http://philip.html5.org/tests/canvas/suite/tests/toDataURL.png.primarycolours.html