This is part of the layout scheme I'm ditching, but just for explanation... the "width: 100% + padding" approach only works because of the `box-sizing: border-box` on the line below this.  Otherwise it would make the content area the entire parent's width and the padding would protrude beyond that, outside of the window.  To do this trick with margins we'd therefore need a `box-sizing: margin-box`, and no such mode exists.