Don't the mouse events give the coordinates in css pixels?
Mouse events come in CSS pixels. A common case for using mouse coordinates is to generate picking rays. Depending on what you divide those coordinates to arrive at device normal coordinates, you might end up in trouble.
* Dividing by canvas style size: OK
* Dividing by canvas.width/height: Was OK, maybe break
* Dividing by drawingBufferWidth/Height: Was OK, maybe break