CSS 动画

<!-- See aria-label: https://mdn.org.cn/en-US/docs/Web/Accessibility/ARIA/Reference/Attributes/aria-label -->
<input
  type="checkbox"
  id="animate"
  aria-label="Toggle the play state of the animation" />
<label for="animate">the animation</label>
<div class="root">
  <i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i
  ><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i
  ><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i
  ><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i
  ><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i><i></i>
  <div class="cloud"></div>
  <div class="ground"></div>
</div>

i {
  display: inline-block;
  height: 16px;
  width: 16px;
  border-radius: 50%;
  animation: falling 3s linear 0s infinite backwards;
  /* Snowflakes are made with CSS linear gradients (https://mdn.org.cn/en-US/docs/Web/CSS/CSS_Images/Using_CSS_gradients) */
  background-image:
    linear-gradient(180deg, transparent 40%, white 40% 60%, transparent 60%),
    linear-gradient(90deg, transparent 40%, white 40% 60%, transparent 60%),
    linear-gradient(45deg, transparent 43%, white 43% 57%, transparent 57%),
    linear-gradient(135deg, transparent 43%, white 43% 57%, transparent 57%);
}
i:nth-of-type(4n) {
  /* Using tree structural pseudo-classes to create randomness - https://mdn.org.cn/en-US/docs/Web/CSS/:nth-of-type */
  height: 30px;
  width: 30px;
  transform-origin: right -30px;
}
i:nth-of-type(4n + 1) {
  height: 24px;
  width: 24px;
  transform-origin: left 30px;
}
i:nth-of-type(4n + 2) {
  height: 10px;
  width: 10px;
  transform-origin: -30px 0;
}
i:nth-of-type(4n + 3) {
  height: 40px;
  width: 40px;
  transform-origin: -50px 0;
}
i:nth-of-type(4n) {
  animation-duration: 5.3s;
  animation-iteration-count: 12;
  transform-origin: -10px -20px;
}
i:nth-of-type(4n + 1) {
  animation-duration: 3.1s;
  animation-iteration-count: 20;
  transform-origin: 10px -20px;
}
i:nth-of-type(4n + 2) {
  animation-duration: 1.7s;
  animation-iteration-count: 35;
  transform-origin: right -20px;
}
i:nth-of-type(3n) {
  animation-delay: 2.3s;
}
i:nth-of-type(3n + 1) {
  animation-delay: 1.5s;
}
i:nth-of-type(3n + 2) {
  animation-delay: 3.4s;
}
i:nth-of-type(5n) {
  animation-timing-function: ease-in-out;
}
i:nth-of-type(5n + 1) {
  animation-timing-function: ease-out;
}
i:nth-of-type(5n + 2) {
  animation-timing-function: ease;
}
i:nth-of-type(5n + 3) {
  animation-timing-function: ease-in;
}
i:nth-of-type(5n + 4) {
  animation-timing-function: linear;
}
i:nth-of-type(11n) {
  animation-timing-function: cubic-bezier(0.2, 0.3, 0.8, 0.9);
}
i:nth-of-type(7n) {
  opacity: 0.5;
}
i:nth-of-type(7n + 2) {
  opacity: 0.3;
}
i:nth-of-type(7n + 4) {
  opacity: 0.7;
}
i:nth-of-type(7n + 6) {
  opacity: 0.6;
  animation-timing-function: ease-in;
  transform-origin: left 10px;
}
i:nth-of-type(7n + 1) {
  opacity: 0.8;
}

.root {
  height: 580px;
  background-color: skyblue;
  border: 1px solid darkgrey;
  position: relative;
  overflow: hidden;
}
.ground,
.cloud {
  position: absolute;
  top: 0;
  right: 0;
  left: 0;
  background-repeat: no-repeat;
}
.cloud {
  width: 100%;
  height: 150px;
  background: white;
  border-radius: 0 0 90px 33% / 0 0 45px 50px;
  box-shadow:
    5px 15px 15px white,
    -5px 15px 15px white,
    0 20px 20px rgb(125 125 125 / 0.5);
  animation:
    clouds ease 5s alternate infinite 0.2s,
    wind ease-out 4s alternate infinite;
}
.ground {
  bottom: 0;
  background-image: linear-gradient(to top, white 97%, 99%, #bbbbbb 100%);
  background-position: center 580px;
  animation: snowfall linear 300s forwards;
  border: 1px solid grey;
  /* Put the ground into a 3D rendering context (because the snow flakes are in a 3d rendering context) */
  transform: translate3d(0, 0, 0);
}

@keyframes snowfall {
  from {
    background-position: center 580px;
  }
  to {
    background-position: center 280px;
  }
}

@keyframes clouds {
  from {
    border-radius: 0 0 90px 33% / 0 0 45px 50px;
  }
  to {
    border-radius: 0 0 40px 50% / 0 0 55px 80px;
  }
}

@keyframes wind {
  from {
    height: 150px;
  }
  to {
    height: 100px;
  }
}

@keyframes falling {
  from {
    transform: translate(0, -50px) rotate(0deg) scale(0.9, 0.9);
  }
  to {
    transform: translate(30px, 600px) rotate(360deg) scale(1.1, 1.1);
  }
}

/* By default, the animations are paused. */
i,
div[class] {
  animation-play-state: paused;
}
/* When the div is hovered, the animation plays. Also,
when the input is checked, the animation coming after the checked checkbox plays */
div:hover *,
input:checked ~ div * {
  animation-play-state: running;
}

/* Change the content of the label that comes right after the input. Included aria-label on the label to improve accessibility. */
input + label::before {
  content: "Play ";
}
input:checked + label::before {
  content: "Pause ";
}