CSS - Vertical & horizontal centering

http://frontendbabel.info/articles/css-tricks-by-wargaming/

by Anton

HTML

<div id="container">
    <div id="inside"></div>
</div>

CSS

#container {
    position: relative;
    width: 400px;
    height: 400px;
    background-color: yellow;
}

#inside {
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    margin: auto;
    width: 120px;
    height: 80px;
    background-color: blue;
}