CSS

css box shadow transform rotate

.item {
  margin-left: 50%;
  position: relative;
  width: 100px;
  height: 100px;
  background-color: red;
}
.item:before {
  z-index: -1;
  position: absolute;
  content: "";
  display: block;
  width: 100px;
  height: 100px;
  top: -30px;
  left: 0;
  background-color: transparent;
  box-shadow: -50px 120px 4px 10px #555;
  -webkit-transform: rotate(10deg);
  -moz-transform: rotate(10deg);
  -o-transform: rotate(20deg);
  -ms-transform: rotate(10deg);
  transform: rotate(10deg);
}