如何让一个div跟图片一样，自适应宽高比例

.html

//https://codepen.io/aghassemi/pen/RpdaKM
<div style="height:320px">

  <div class="parent">
    <div class="wrapper">
      <!-- To change aspect ratio, change SVG's width/height -->
      <img class="resizer" src='data:image/svg+xml;utf8,<svg height="2px" width="1px" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1"></svg>'></img>
      <div class="child">Child maintains aspect ratio as parent's height changes</div>
    </div>
  </div>

</div>

<div style="float:left;">
  <h2>How does this work?</h2>
  <p>`img` can update its width based on its height while mainating the aspect ratio as defined by its intrinsic size. Using an SVG we create an image with intrinsic aspect ratio we want and rely on the browser to resize the width when height changes.</p>
</div>

<style>
  .parent {
  padding: 10px;
  height: 300px;
  background: green;
  animation-name: resize;
  animation-duration: 4s;
  animation-iteration-count: infinite;
  animation-direction: alternate;
}

.resizer {
  display: block;
  min-height:100%;
  width: auto;
  height: auto;
}

.wrapper {
  display: inline-block;
  height: 100%;
  position: relative;
  /* Chrome bug work-around. 
   * See https://bugs.chromium.org/p/chromium/issues/detail?id=707151
  */
/*   padding-right: 0.0001%; */
}

.child {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background: red;
}
@keyframes resize {
    from {height: 300px;}
    to {height: 150px;}
}
  
</style>