Андрей Ноготков

.box {
  position: relative;
  width: 100%;
  height: 500px;
}
.box:after {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-image: linear-gradient(to bottom right, transparent 50%, white 50%, white 50.1%, transparent 0);
}