<div class="elemento"></div>
body {
  margin: 0;
  padding: 0;
}
.elemento {
  background-color: grey;
  height: 100vh;
  width: 100%;
  &:after {
    content: "";
    display: block;
    position: absolute;
    bottom: 0;
    left: 0;
    width: 0;
    height: 0;
    border-bottom: 20vh solid rgba(253, 99, 77, 0.4);
    border-left: 100vw solid transparent;
    z-index: 10;
  }
}
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