console 命令行工具 X clear

console

<body>
  <div class="a">
    <div class="b"></div>
  </div>
</body>

body{
  display: table;
  width:100%;
  height: 100%;
  position: fixed;
  z-index: 9998;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
}

.a{
  display: table-cell;
  vertical-align: middle;
}

.b{
  width: 100px;
    height: 1200px;
  background: black;
  margin: 0 auto;
}