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;
}