div#header{
  width: 960px;
  height: 200px;
  left: 50%;
  margin-left: -480px;
  position: absolute;
 }

div#content{
 width: 960px;
 border: 1px solid black;
 left: 50%;
 margin-left: -480px;
 position: absolute;
 margin-top: 200px;
 }

div#footer{
  width: 960px;
  height: 30px;
  left: 50%;
  margin-left: -480px;
  position: absolute;
  bottom: 10px; /*with this i've div fixed to the bottom of web-browsers' window */
  clear: both;
  }

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