使用弹性盒子，居中变的很简单，只想要设置 margin: auto; 可以使得弹性子元素在两上轴方向上完全居中 - 在线编辑器

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <style>
    .flex-container {
      display: -webkit-flex;
      display: flex;
      width: 400px;
      height: 250px;
      background-color: lightgrey;
    }

    .flex-item {
      background-color: cornflowerblue;
      width: 75px;
      height: 75px;
      margin: auto;
    }
  </style>
</head>
<body>

  <div class="flex-container">
    <div class="flex-item">Perfect centering!</div>
  </div>

</body>
</html>
​x
 
<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <style>
    .flex-container {
      display: -webkit-flex;
      display: flex;
      width: 400px;
      height: 250px;
      background-color: lightgrey;
    }
​
    .flex-item {
      background-color: cornflowerblue;
      width: 75px;
      height: 75px;
      margin: auto;
    }
  </style>
</head>
<body>
​
  <div class="flex-container">
    <div class="flex-item">Perfect centering!</div>
  </div>
​
</body>
</html>