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:;
     margin:;
     top:;
     left:;
     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：