body {
  background: -webkit-linear-gradient(303deg, #980587, #00c3dd);
  background: linear-gradient(147deg, #980587, #00c3dd);
  background-size: 1000% 1000%;
  animation: GradientAnimation 20s ease infinite;
}

@keyframes GradientAnimation {
  0%   { background-position: 0% 50% }
  50%  { background-position: 100% 50% }
  100% { background-position: 0% 50% }
}

External CSS

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External JavaScript

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