This is the easiest way to get a border down the side of your browser. You just make a very wide, but very thin gif with a left border the colour you want.
For example, the gif that made this border is just 3 pixels high and about 1500 pixels wide. That's wider than most users will have as a screen width, so they'll never see it tile horizontally. but they will see the vertical tiling - hence the border you see down the left hand side.
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