#box1{position:absolute; width:639px; top:0; right:0;}
/*i div devono sempre avere un background altrimenti IE...*/
#box1 div{position:absolute;background-color:#ffffff;width:700px;height:175px;}
.mask{position:relative;width:639px;height:175px;overflow:hidden;}
.mask img{position:absolute;display:block;border:none;}