.outer-div {
height: 500px;
width: 100%;
position: relative;
background-color: bisque;
}
.inner-div {
position: absolute;
top:50%;
left:50%;
transform:translateY(-50%) translateX(-50%);
background-color: aquamarine;
}
4.宽高已知，绝对定位：
.outer-div {
height: 500px;
width: 100%;
position: relative;
background-color: bisque;
}
.inner-div {
position: absolute;
top:calc( 50% - 25px);
left:calc( 50% - 50px);
width:100px;
height:50px;
background-color: aquamarine;
}
