My guess is that this would be caused by rendering text into a canvas and then scaling the canvas to fit the window. That example is rendering at 640x960 regardless of the screen size.
Text rendering, manipulation, and searching, is one of the key issues holding back canvas adoption as a whole. The DOM is still king here.
Text rendering quality is not as bad as it used to be, pretty much on par with DOM. In any event, if you do not scale canvas resolution by devicePixelRatio, the result will be blurry on most devices these days.