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If you have an angle of 0 and a radius > 0, the mac and win port handle arcTo different.
The mac port just adds (x1,y1) to the current path and draws a line of (x0,y0) to (x1,y1). http://philip.html5.org/demos/canvas/arcto-inf.png
The win port draws to parallel lines to the infity. One throug (x1,y1)->(x0,y0) and the second parallel to this line with the distance of the radius.