Have you ever loaded a background that needs to match up with the position of objects, and found it doesnt after you designed it so it should.
Ive seen this a few times.
So now in my editor I ignore the fact that the background is four pixels higher, because when it publishes, it all lines up as it should.
If I add one pixel to my background at the top, it suddenly jumps down to be one pixel lower than it should.
You can see this best here, with NOF in front and the published view behind.
Click image for larger version. 

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Also live here: http://www.solarlightcap.com/html/certs.html

(Rather pleased with myself for getting the screen wide block style working which is all the fashion nowadays )