CSS warped shadow implementation code

<ul class="box">
    <li><img src="......" alt=""></li>
    <li><img src="......" alt=""></li>
    <li><img src="......" alt=""></li>
</ul>

ul,li {
    list-style:none;
}
.box {
    width: 980px;
    height: auto;
    clear: both;
    overflow: hidden;
    margin: 20px auto;
}
.box li {
    width: 300px;
    height: 210px;
    position: relative;
    background: #fff;
    float: left;
    margin: 20px 10px;
    border: 2px solid #efefef;
    box-shadow: 0 1px 4px rgba(0, 0, 0, .27), 0 0 60px rgba(0, 0, 0, 0.1) inset;
}
.box li img {
    display: block;
    width: 290px;
    height: 200px;
    margin: 5px;
}
.box li:before, .box li:after {
    content: '';
    position: absolute;
    z-index: -2;
    width: 80%;
    height: 80%;
    bottom: 8px;
    background: transparent;
    box-shadow: 0 8px 26px rgba(0, 0, 0, 0.6);
}
.box li:before {
    left: 7%;
    transform: skewX(-12deg) rotate(-4deg);
}
.box li:after {
    right: 7%;
    transform: skewX(12deg) rotate(4deg);
}