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

Answer 1

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

or simply

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