定位

#position1{
width: 100px;
height: 100px;
background-color: blue;
position: absolute;
left: 20px;
top: 20px;
z-index: 3;
}
#position2{
width: 100px;
height: 100px;
background-color: aqua;
position: fixed;
left: 10px;
top: 10px;
z-index: 2;
}
#position3{
width: 100px;
height: 100px;
background-color: red;
position: relative;
left: 20px;
top: 20px;
z-index: 1;
}