I have this page: http://jsfiddle.net/minitech/yU3aj/show/
If you look in the source, the CSS defines that the <header> should have side padding of 135px on each side, and the content should have 135px margin on each side. Why does the content end up having double that in spacing?

It doesn’t. You’re floating elements in the
<header>and not clearing them.Add
clear: both;to your styles for the contentdivand it will move below the header and to the left.