How to align a to the middle (horizontally/width) of the page [duplicate]

Answer 1

body, html {
    display: table;
    height: 100%;
    width: 100%;
}
.container {
    display: table-cell;
    vertical-align: middle;
}
.container .box {
    width: 100px;
    height: 100px;
    background: red;
    margin: 0 auto;
}

http://jsfiddle.net/NPV2E/

"width:100%" for the "body" tag is only for an example. In a real project you may remove this property.