.longshadow {
  position: relative;
}
.longshadow:before, .longshadow:after {
  bottom: -10vw;
  left: 0;
  -webkit-transform-origin: 0 0;
          transform-origin: 0 0;
  content: '';
  position: absolute;
  width: 70.6%;
  height: 10.1vw;
  background: #000;
  opacity: .1;
  -webkit-transform: rotate(-45deg);
          transform: rotate(-45deg);
  z-index: -1;
  box-shadow: 0 10vw #000, 0 20vw #000, 0 30vw #000, 0 40vw #000, 0 50vw #000, 0 60vw #000, 0 70vw #000, 0 80vw #000, 0 90vw #000, 0 100vw #000, 0 110vw #000, 0 120vw #000, 0 130vw #000, 0 140vw #000, 0 150vw #000;
}
.longshadow:after {
  top: 0;
  right: 0;
  left: auto;
  width: var(--longshadow-h);
  -webkit-transform-origin: 100% 0;
          transform-origin: 100% 0;
}

body {
  background: #59e;
  height: 600vh;
  margin: 0;
  padding: 0;
}

div {
  background: #fff;
  top: 20vmin;
  width: 60vmin;
  height: 60vmin;
  margin: auto;
}