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<div id="container">
    <div id="inner">
        Содержимое
    </div>
</div>
#container {    
    background:#6BB533;
    height:400px;
    width:400px;
    position: relative;
    
}
#inner {
    background:#fcfcfc;
    height:100px;
    width:100px;
    margin:auto;
    position: absolute;
    top: 0; left: 0; bottom: 0; right: 0;    
}