The "seven properties" of horizontal formatting are: margin-left, border-left, padding-left, width, padding-right, border-right, and margin-right. Padding and margin is set to 0 by default. Width is set to auto. This means the block element will try expend as much as possible into its containing content space. /p>
If width is auto, whether margin-left and margin-right is set to auto or just 0 has not effect. Because either way, their computed value will be the same 0. But if width is has a value, and margine-left and margin-right is set to auto, then that margin will expand or contracted automatically. If both margin is set to auto, and the width has a value, the the content of the block will be set to horizonal center.