align absolute div center

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

position: absolute;
margin-left: auto;
margin-right: auto;
left: 0;
right: 0;

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

.container {
  left:0;
  right:0;

  margin-left: auto;
  margin-right: auto;

  position: absolute;
  width: 40%;

  outline: 1px solid black;
  background: white;
}