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: 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>

效果如下：