第一个

body:before {
    content: "";
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    z-index: -520;
    pointer-events: none
}

body:before {
    background: linear-gradient(90deg,#f795331a,#f370551a 15%,#ef4e7b1a 30%,#a166ab1a,#5073b81a,#1098ad1a,#07b39b1a 86%,#6dba821a)
}

第二个

body {
  margin: 0;
  min-height: 100vh;
  background-color: #e493d0;
  background-image: 
    radial-gradient(closest-side, rgba(235, 105, 78, 1), rgba(235, 105, 78, 0)),
    radial-gradient(closest-side, rgba(243, 11, 164, 1), rgba(243, 11, 164, 0)),
    radial-gradient(closest-side, rgba(254, 234, 131, 1), rgba(254, 234, 131, 0)),
    radial-gradient(closest-side, rgba(170, 142, 245, 1), rgba(170, 142, 245, 0)),
    radial-gradient(closest-side, rgba(248, 192, 147, 1), rgba(248, 192, 147, 0));
  background-size: 
    130vmax 130vmax,
    80vmax 80vmax,
    90vmax 90vmax,
    110vmax 110vmax,
    90vmax 90vmax;
  background-position:
    -80vmax -80vmax,
    60vmax -30vmax,
    10vmax 10vmax,
    -30vmax -10vmax,
    50vmax 50vmax;
  background-repeat: no-repeat;
  animation: 10s movement linear infinite;
}

@keyframes movement {
  0%, 100% {
    background-size: 
      130vmax 130vmax,
      80vmax 80vmax,
      90vmax 90vmax,
      110vmax 110vmax,
      90vmax 90vmax;
    background-position:
      -80vmax -80vmax,
      60vmax -30vmax,
      10vmax 10vmax,
      -30vmax -10vmax,
      50vmax 50vmax;
  }
  25% {
    background-size: 
      100vmax 100vmax,
      90vmax 90vmax,
      100vmax 100vmax,
      90vmax 90vmax,
      60vmax 60vmax;
    background-position:
      -60vmax -90vmax,
      50vmax -40vmax,
      0vmax -20vmax,
      -40vmax -20vmax,
      40vmax 60vmax;
  }
  50% {
    background-size: 
      80vmax 80vmax,
      110vmax 110vmax,
      80vmax 80vmax,
      60vmax 60vmax,
      80vmax 80vmax;
    background-position:
      -50vmax -70vmax,
      40vmax -30vmax,
      10vmax 0vmax,
      20vmax 10vmax,
      30vmax 70vmax;
  }
  75% {
    background-size: 
      90vmax 90vmax,
      90vmax 90vmax,
      100vmax 100vmax,
      90vmax 90vmax,
      70vmax 70vmax;
    background-position:
      -50vmax -40vmax,
      50vmax -30vmax,
      20vmax 0vmax,
      -10vmax 10vmax,
      40vmax 60vmax;
  }
}

第三个

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}
body {
  overflow:scroll;
  overflow-x:hidden;
  width: 100vw;
  height: 100vh;
  background: linear-gradient(
          135deg,
          hsl(170deg, 80%, 70%),
          hsl(190deg, 80%, 70%),
          hsl(250deg, 80%, 70%),
          hsl(320deg, 80%, 70%)
  );
  background-size: 200% 200%;
  animation: gradient-move 15s ease alternate infinite;
}

@keyframes gradient-move {
  0% {
    background-position: 0% 0%;
  }
  100% {
    background-position: 100% 100%;
  }
}