Look for whatever code or CSS file sets the parameters for "canvas0" and "canvas1". I see that they are
position: absolute with
left: 0, which should place them hard left within their parent, which appears to be an anonymous <div> of the same size as the two canvases. Its positioning is "relative" to its natural position, but there are no offsets. If I understand it, that should leave your two canvases at the left edge of the screen. The first thing I'd try is changing
document.write('<div style="position:relative;width:'+trans_width+';height:'+trans_height+';overflow:hidden">
to
document.write('<div style="position:absolute; left:0; margin-left: auto; margin-right: auto; width:'+trans_width+';height:'+trans_height+';overflow:hidden">
See if matters improve at all.
