How can I center a div within another div? [duplicate]

Answer 1

#main_content {
    width: 400px;
    margin: 0 auto;
    min-height: 300px;
    height: auto;
    background-color: #2185C5;
    position: relative;
}

#container {
    width: 50%;
    height: auto;
    margin: 0 auto;
    background-color: #CCC;
    padding: 10px;
    position: relative;
}

Try this. It tested OK. There is a live check on jsfiddle.