You have probably seen these before, at least in a pixel version, then you know how to look to see the 3d picture. Since it's hard to explain how to look, the best description I can give you is to focus slightly behind the screen, at first the text will become a blur, than a hight differance will occure. Try to focuse on the heigher layer.
For this to work, you need a fixed font, like courier. This is what you get if you have sane settings in your browser. The text should make a rectangle. If not paste the text into your favourite font-aware editor and pick the font courier.

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