Edit in JSFiddle


              
            
          
            
              
                
<div class="parentBox">
     <div class="childBox">Box</div>
</div>
.parentBox{
     position:relative;
     width:100%;
     border:solid 1px #f00;
}
.childBox{
     position:absolute;
     height:100px;
     background-color:#0f0;
}