One way you can do it is by styling the pesedo elements ::before
and ::after
like this:
div {
width: 300px;
height: 400px;
background: #FFF;
margin: 40px auto;
position: relative;
}
div::before,
div::after {
z-index: -1;
position: absolute;
content: "";
width: 50%;
height: 50%;
right: 10px;
top: 10px;
max-width: 300px;
background: #777;
box-shadow: 15px 0px 10px #777;
transform: rotate(3deg);
}
div::after {
transform: rotate(-3deg);
top: calc(50% - 10px);
}
<div></div>
This see pen for more shadow styling.