Centering a percent-based div

Question

Recently, a client asked that his site be percent-based rather than pixel-based. The percent was to be set to 80%. As you guys know, it is very easy to center the container if it is pixel-based but how do you center a percent-based main container?

#container {   width:80%;   margin:0px auto; } 

That does not center the container :(

Answer 1

The margin property supports percentage values:

margin-left: 10%; margin-right: 10%; 

Answer 2

#container {   width:80%;   position:absolute;   margin-left:-40%;   left:50%; } 

or simply

#container {   width:80%;   margin-left:10%; }