Basically in IE6, NS6/7 Opera and Mozilla the width of an element is it's contents width with border and padding on top of that, as it is specified.
However in IE5 a bug means that width is taken to be the width of the content plus padding plus borders. That means if you set the width on an element and give it a 7 pixel border in IE5 it will appear 14 pixels narrow than you expect (7 on the left and 7 on the right).
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