The problem is highlighted; 
http://www.imperialviolet.org/2009/09/02/anti-aliased-clipping.html
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One solution is built into CANVAS;
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You should see two black boxes next to eachother;
* The 1st one should have a sub-pixel antialiasing line.
	- using ctx.globalCompositeOperation = "source-over"
* The 2nd one should be joined into a perfect square
	- using ctx.globalCompositeOperation = "lighter"
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Lighter mode is used to join the pixels,
the sub-pixels of the antialiasing should always 
add up to 1, if the shapes really fit together perfectly!
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Both "lighter" and "darker" mode works, but only Safari supports "darker".
Both modes seem to add the pixels together, when the combined alpha
of the overlaying pixels is == 1.
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Mr. Doob points out it does not work in Chrome for Windows or Linux.
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It currently works in: 
	OSX: Firefox, Chrome, Safari, Opera
	Windows: Opera, IE9, Safari
	Linux: Firefox