下雪效果

HTML

<div class="snow-container">
  <div class="snow foreground"></div>
  <div class="snow foreground layered"></div>
  <div class="snow middleground"></div>
  <div class="snow middleground layered"></div>
  <div class="snow background"></div>
  <div class="snow background layered"></div>
</div>

CSS

.snow-container {
  position: fixed;
  top:0;
  left:0;
  right: 0;
  bottom:0;
  overflow: hidden;
  z-index: 2;
  pointer-events: none;
  background-color: none;
}

.snow {
  display: block;
  position: absolute;
  z-index: 2;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  pointer-events: none;
  -webkit-transform: translate3d(0, -100%, 0);
          transform: translate3d(0, -100%, 0);
  -webkit-animation: snow linear infinite;
          animation: snow linear infinite;
}
.snow.foreground {
  background-image: url("https://dl6rt3mwcjzxg.cloudfront.net/assets/snow/snow-large-075d267ecbc42e3564c8ed43516dd557.png");
  -webkit-animation-duration: 15s;
          animation-duration: 15s;
}
.snow.foreground.layered {
  -webkit-animation-delay: 7.5s;
          animation-delay: 7.5s;
}
.snow.middleground {
  background-image: url(https://dl6rt3mwcjzxg.cloudfront.net/assets/snow/snow-medium-0b8a5e0732315b68e1f54185be7a1ad9.png);
  -webkit-animation-duration: 20s;
          animation-duration: 20s;
}
.snow.middleground.layered {
  -webkit-animation-delay: 10s;
          animation-delay: 10s;
}
.snow.background {
  background-image: url(https://dl6rt3mwcjzxg.cloudfront.net/assets/snow/snow-small-1ecd03b1fce08c24e064ff8c0a72c519.png);
  -webkit-animation-duration: 30s;
          animation-duration: 30s;
}
.snow.background.layered {
  -webkit-animation-delay: 15s;
          animation-delay: 15s;
}

@-webkit-keyframes snow {
  0% {
    -webkit-transform: translate3d(0, -100%, 0);
            transform: translate3d(0, -100%, 0);
  }
  100% {
    -webkit-transform: translate3d(15%, 100%, 0);
            transform: translate3d(15%, 100%, 0);
  }
}

@keyframes snow {
  0% {
    -webkit-transform: translate3d(0, -100%, 0);
            transform: translate3d(0, -100%, 0);
  }
  100% {
    -webkit-transform: translate3d(15%, 100%, 0);
            transform: translate3d(15%, 100%, 0);
  }
}