.frame {
display: inline-block;
position: relative;
}
.frame:after {
position: absolute;
display: block;
content: "";
top: 0;
left: 0;
width: 100%;
height: 100%;
background: #fff;
box-shadow:
inset 0 0 20px #fff /* 背景色と同じ色 */,
inset 0 0 20px #fff,
inset 0 0 20px #fff,
inset 0 0 20px #fff;
}
