FCC---Animate Multiple Elements at Variable Rates---还可以改循环时间，达到不同律动频率的效果

In the previous challenge, you changed the animation rates for two similarly animated elements by altering their @keyframes rules.

You can achieve the same goal by manipulating the animation-duration of multiple elements.

In the animation running in the code editor, there are three "stars" in the sky that twinkle at the same rate on a continuous loop.

To make them twinkle at different rates, you can set the animation-duration property to different values for each element.

练习题目：

In the previous challenge, you changed the animation rates for two similarly animated elements by altering their @keyframes rules.

You can achieve the same goal by manipulating the animation-duration of multiple elements.

In the animation running in the code editor, there are three "stars" in the sky that twinkle at the same rate on a continuous loop. To make them twinkle at different rates, you can set the animation-duration property to different values for each element.

练习代码：

<style>
  .stars {
    background-color: white;
    height: 30px;
    width: 30px;
    border-radius: 50%;
    animation-iteration-count: infinite;
  }

  .star-1 {
    margin-top: 15%;
    margin-left: 60%;
    animation-duration: 1s;
    animation-name: twinkle;
  }

  .star-2 {
    margin-top: 25%;
    margin-left: 25%;
    animation-duration: 0.9s;
    animation-name: twinkle;
  }

  .star-3 {
    margin-top: 10%;
    margin-left: 50%;
    animation-duration: 1.1s;
    animation-name: twinkle;
  }

  @keyframes twinkle {
    20% {
      transform: scale(0.5);
      opacity: 0.5;
    }
  }

  #back {
    position: fixed;
    padding: 0;
    margin: 0;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: linear-gradient(black, #000099, #66c2ff, #ffcccc, #ffeee6);
  }
</style>

<div id="back"></div>
<div class="star-1 stars"></div>
<div class="star-2 stars"></div>
<div class="star-3 stars"></div>

效果如下：