css如何让边框透明

div {    width: 100px;   

height:100px;    border-top:10px solid  rgba(200,200,200,0.25);    border-right:10px solid  rgba(200,200,200,0.5);    border-bottom: 10px solid  rgba(200,200,200,0.75);    border-left:10px solid  rgba(200,200,200,1);}