废话不多说，直接上代码
父级元素position属性设置为 relative
body {
height:100%;
width:100%;
}
div-one {
position:relative;
height:100px;
width:100px;
}
子元素css
div-one-content {
position: absolute;
height:100px;
width:100px;
background-color: #cbcb04;
top: 50%;
left: 50%;
margin-left: -50px;
margin-top: -50px;
}