why the extra 10 pixels in this JSfiddle?

Can someone tell me why I have to subtract an extra 10 pixels from th开发者_开发技巧e positioning in this JSFiddle?

http://jsfiddle.net/MPFyt/

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It's because the jsfiddle window has 10px of padding around it.

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It's just the CSS reset that's tripping you up. http://jsfiddle.net/mattball/YQPsz/