1. 效果

2. html

<div class="count">
  <span>3</span>
  <span>2</span>
  <span>1</span>
</div>

3. css

body {
  width: 100vw;
  height: 100vh;
  overflow: hidden;
  display: flex;
  justify-content: center;
  align-items: center;
}

.count {
  position: relative;
  width: 180px;
  height: 180px;
  font-family: Consolas, Monaco, monospace;
  font-size: 100px;
  border: 5px solid #333333;
  border-radius: 50%;
  overflow: hidden;
}

.count span {
  display: block;
  width: 100%;
  height: 180px;
  line-height: 180px;
  text-align: center;
  animation: count 5s steps(5, end) forwards, shark 1s .8s 5;
}

.count::after {
  content: 'Go!';
  position: absolute;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
  opacity: 0;
  font-size: 70px !important;
  animation: go .5s 3s forwards;
}

@keyframes count {
  to {
    transform: translateY(calc(-5 * 180px));
  }
}

@keyframes shark {
  0% {
    opacity: 1;
  }

  20% {
    opacity: 0;
    font-size: 40px;
  }
}

@keyframes go {
  to {
    font-size: 70px;
    opacity: 1;
  }
}