用CSS和Bootstrap图标制作上下跳动的指示箭头动画效果

<a href="#" class="scroll-down">
  <span> <i class="glyphicon glyphicon-chevron-down"></i> </span>
</a>

/*向下滑动的动画效果*/
@-webkit-keyframes drop {
  0%   { top:0px;  opacity: 0;}
  30%  { top:10px; opacity: 1;}
  100% { top:25px; opacity: 0;}
}
@keyframes drop {
  0%   { top:0px;  opacity: 0;}
  30%  { top:10px; opacity: 1;}
  100% { top:25px; opacity: 0;}
}

/*应用动画，添加按钮效果*/
.scroll-down {
    border: 2px solid #bbb;
    border-radius: 50%;
    margin: 0 auto;
    height: 50px;
    width: 50px;
    display: block;
    text-align: center;
    z-index: 10;
    -webkit-transition: all 0.125s ease-in-out 0s;
    -moz-transition: all 0.125s ease-in-out 0s;
    -ms-transition: all 0.125s ease-in-out 0s;
    -o-transition: all 0.125s ease-in-out 0s;
    transition: all 0.125s ease-in-out 0s;
}
.scroll-down span {
    position: relative;
    color: #bbb;
    font-size: 24px;
    -webkit-animation-name: drop;
    -webkit-animation-duration: 1s;
    -webkit-animation-timing-function: linear;
    -webkit-animation-delay: 0s;
    -webkit-animation-iteration-count: infinite;
    -webkit-animation-play-state: running;
    animation-name: drop;
    animation-duration: 1s;
    animation-timing-function: linear;
    animation-delay: 0s;
    animation-iteration-count: infinite;
    animation-play-state: running;
}