让弹框在屏幕垂直居中

html
<div class="zhehzao">
  <div class="tankaung">在屏幕水平垂直居中</div>
<div>
css
.zhezhao{
    width: 100%;
    height: 100%;
    position: fixed;
    z-index: 10000;
    top:0;
    background: rgba(0,0,0,.5);
    display: flex;
    align-items: center;
}
.tankaung{
    width: 100px;
    height: 100px;
    margin: 0 auto
}