1.
.fatherBox{ width:500px; height:500px; position:relative; }
.sonBox{ width:200px; height:200px; position:absolute; top:0; botton:0; left:0; right:0; margin:auto; }
2.
.fatherBox{ width:500px; height:500px; position:relative; } .sonBox{ width:200px; height:200px; position:absolute; top:50%; left:50%; margin-top:-100px; margin-left:-100px; }
3.
.fatherBox{ width:500px; height:500px; position:relative; } .sonBox{ position:absolute; top:50%; left:50%; transform:translate(-50%,-50%) }
4.
.fatherBox{ width:500px; height:500px; display:flex; justify-content:center; align-items:center; } .sonBox{ width:200px; height:200px; }