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CSS

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JavaScript

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HTML 

              
                <div class="colour-1"></div>
<div class="colour-2"></div>
<div class="colour-3"></div>

<!-- The below code is not needed -->
<div class="aurora">
  <div class="aurora__container">
    <h1>Aurora Example</h1>
    <p>Example of using CSS to create and Aurora style effect.</p>
  </div>
</div>
!

CSS 

              
                // - - - - - - - - - - - - - - -
// Code for the Aurora effect
// - - - - - - - - - - - - - -
body {
  background: #1c1ca2;
}

div {
  position: absolute;
  opacity: 0.75;
}

@function shadow($color) {
  @return 0 0 45vmax 45vmax #{$color};
}

@keyframes hueRotate {
  to {
    filter: hue-rotate(360deg);
  }
}

@keyframes colour-1 {
  0% {
    top: 0vh;
    left: 50vw;
  }
  25% {
    left: 0vw;
  }
  50% {
    top: 100vh;
  }
  75% {
    left: 100vw;
  }
  100% {
    top: 0vh;
    left: 50vw;
  }
}

@keyframes colour-2 {
  0% {
    top: 50vh;
    left: 100vw;
  }
  25% {
    top: 100vh;
  }
  50% {
    left: 0vw;
  }
  75% {
    top: 0vh;
  }
  100% {
    top: 50vh;
    left: 100vw;
  }
}

@keyframes colour-3 {
  0% {
    top: 100vh;
    left: 50vw;
  }
  25% {
    left: 100vw;
  }
  50% {
    top: 0vh;
  }
  75% {
    left: 0vw;
  }
  100% {
    top: 100vh;
    left: 50vw;
  }
}

.colour-1 {
  box-shadow: shadow(purple);
  animation: hueRotate 10s 0s linear infinite, colour-1 19s 0s linear infinite;
}

.colour-2 {
  box-shadow: shadow(blue);
  animation: hueRotate 15s 0s linear infinite, colour-2 25s 0s linear infinite;
}

.colour-3 {
  box-shadow: shadow(red);
  animation: hueRotate 20s 0s linear infinite, colour-3 15s 0s linear infinite;
}

// - - - - - - - - - - - - - - - - - - - - -
// Extra code for the text box (not needed)
// - - - - - - - - - - - - - - - - - - - - -
@keyframes fadeOut {
  to {
    opacity: 0;
  }
}
.aurora,
.aurora > * {
  z-index: 2;
  position: static;
  opacity: 1;
  display: grid;
  place-content: center;
  width: 100vw;
  height: 100vh;
}

.aurora__container {
  animation: fadeOut 3s 5s linear 1 forwards;
  display: block;
  height: auto;
  width: auto;
  max-width: 600px;
  padding: clamp(20px, 5vw, 60px);
  margin: clamp(8px, 4vw, 24px);
  background-color: #ffffff;
  color: #00000097;
  border-radius: 10px;
  box-shadow: 0px 12px 24px -5px rgba(0, 0, 0, 0.2);
  mix-blend-mode: soft-light;

  h1 {
    text-align: center;
    font-family: sans-serif;
    font-size: clamp(25px, 5vw, 50px);
    line-height: 1.2;
    padding-bottom: 16px;
    font-weight: bold;
    -webkit-text-stroke-width: thin;
    -webkit-text-stroke: black;
  }

  p {
    font-weight: bold;
    text-align: center;
    font-family: sans-serif;
    font-size: clamp(16px, 4vw, 30px);
    line-height: 1.2;
    -webkit-text-stroke-width: thin;
    -webkit-text-stroke: black;
  }
}
!

JS
!
999px

Console