<div></div>
html,body { /*这里没用*/
width: 100%;
height: 100%;
margin: 0;
padding: 0;
}
div {
width:100px;
height:100px;
border-radius:50%;
background-color: turquoise;
/*下面的只是垂直居中而已*/
margin: 0 auto;
position: relative;
top: 50%;
margin-top: -50px;
}