margin 0 auto

1Specifying 0 tells the browser to set the top and bottom margin to 0.
2
3Specifying auto tells the browser to automatically determine the left
4and right margins equally.
5
6It's the same as:
7
8margin-top:0;
9margin-bottom:0;
10margin-left:auto;
11margin-right:auto;
12
13

1/* Apply to all four sides */
2margin: 1em;
3margin: -3px;
4
5/* vertical | horizontal */
6margin: 5% auto;
7
8/* top | horizontal | bottom */
9margin: 1em auto 2em; 
10
11/* top | right | bottom | left */
12margin: 2px 1em 0 auto;
13
14/* Global values */
15margin: inherit;
16margin: initial;
17margin: unset;