center absolute

#content {
  position: absolute; 
  left: 0; 
  right: 0; 
  margin-left: auto; 
  margin-right: auto; 
  width: 100px; /* Need a specific value to work */
}

position: absolute;
margin-left: auto;
margin-right: auto;
left: 0;
right: 0;
text-align: center;