JavaScript结合CSS动画实现弹窗效果

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>简单弹窗</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }
    .pop {
      width: 400px;
      height: 300px;
      position: fixed;
      bottom: 0;
      right: 0;
      display: none;
      animation: pop 1s ease-in-out 0s;
    }
    @keyframes pop {
      from {
        height: 0;
      }
      to {
        height: 300px;
      }
    }
    .down {
      width: 400px;
      height: 0;
      position: fixed;
      bottom: 0;
      right: 0;
      display: block;
      animation: out 1s ease-in-out;
    }
    @keyframes out {
      from {
        height: 300px;
      }
      to {
        height: 0;
      }
    }
    .img1 {
      width: 400px;
      height: 300px;
      vertical-align: top;
    }
  </style>
</head>
<body>
<div class="pop" id="pop">
  <img src="images/01.jpg" alt="" class="img1">
</div>
</body>
<script>
  window.onload = function () {
    timer = window.setInterval(imgBlock, 2000);
  };
  function imgBlock() {
    var pop = document.getElementById('pop');
    pop.style.display = 'block';
    timer2 = window.setInterval(imgNone,5000);
  }
  function imgNone() {
    var pop = document.getElementById('pop');
    pop.className = 'down';
    clearInterval(timer);
    clearInterval(timer2);
  }
</script>
</html>