*{
margin:0;
padding:0;
}
/*第一種方案*/
/*div{
width: 100px;
height: 100px;
border:1px solid blue;
position: absolute;
left: 50%;
top: 50%;
margin: -50px 0 0 -50px;
}*/
/*第二種方案
div{
width: 100px;
height: 100px;
border: 1px solid blue;
position: absolute;
left: 0;
top: 0;
right: 0;
bottom: 0;
margin:auto;
}*/
/*第三種方案
div{
position: fixed;
top: 50%;
left: 50%;
background: #000;
width: 50%;
height: 50%;
transform: translateX(-50%) translateY(-50%);
}*/
body
<div></div>
