css实现垂直水平居中

#app{
    background:green;
    width:500px;
    height:500px;
    position:relative;
}
#box{
    background:red;
    position:absolute;
    left:50%;
    top:50%;
    transform:translate(-50%,-50%);
} 

#app {
  display: flex;
  justify-content: center;
  align-items: center;
  width: 500px;
  height: 500px;
}
#box{
  width: 100px;
  height: 100px;
}

<div id=‘app‘></div>
<div id="box"></div>

#app{
    background:green;
    position:fixed;
    top:0;left:0;right:0;bottom:0;
}
#box{
    background:red;
    position:absolute;
    left:0;
    top:0;
    right:0;
    bottom:0;
    width:300px;
    height:300px;
    margin:auto;
}