body {
  margin: 0;
  width: 30vw;
  height: 100vh;
  background-color:#d50000;
  background-image: 
    linear-gradient( 
      90deg, 
      #000000F0, 
      #FF0000A0 var(--lpos), 
      #000000C0),
    repeating-linear-gradient(
      45deg,
      rgba(0,0,0,0.8),
      rgba(0,0,0,0.8) 100px,
      transparent 0px,
      transparent 200px
    ),
    repeating-linear-gradient(
      -45deg,
      rgba(0,0,0,0.5),
      rgba(0,0,0,0.5) 100px,
      transparent 0px,
      transparent 200px
    );
  background-size: 100%;
  animation: move 10s infinite both;
}

@keyframes move{
  0% {
    --lpos: 0%;
  }
  25% {
    --lpos: 75%;
  }
  50% {
    --lpos: 50%;
  }
  75% {
    --lpos: 25%;
  }
  100% {
    --lpos: 100%;
  }
}

External CSS

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External JavaScript

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