FCC---Animate Elements at Variable Rates----一闪一闪亮晶晶，不同的闪动节奏

There are a variety of ways to alter the animation rates of similarly animated elements.

So far, this has been achieved by applying an animation-iteration-count property and setting @keyframes rules.

To illustrate, the animation on the right consists of two "stars" that each decrease in size and opacity at the 20% mark in the @keyframes rule, which creates the twinkle animation.

You can change the @keyframes rule for one of the elements so the stars twinkle at different rates.

练习题目：

Alter the animation rate for the element with the class name of star-1 by changing its @keyframes rule to 50%.

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-name: twinkle-1;
     animation-duration: 1s;
   }

   .star-2 {
     margin-top: 25%;
     margin-left: 25%;
     animation-name: twinkle-2;
     animation-duration: 1s;
   }

   @keyframes twinkle-1 {
     50% {
       transform: scale(0.5);
       opacity: 0.5;
     }
   }

   @keyframes twinkle-2 {
     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>

效果如下：