canvas is inconsistent on webkit too. On iOS you need to create the element at twice the size because canvas size == backing size, while Safari/Mac "transparently" does this (and breaks direct pixel access). Chrome/Mac hasn't made up their mind which model to follow.

You need to look at both window.devicePixelRatio and canvasContext.webkitBackingStorePixelRatio to detect if you need to do manual pixel-doubling.