<div class="test0"></div>
<div class="test1"></div>
.test0{
width: 10px;
height: 10px;
position: fixed;
top: 0;
left: 0;
background: yellow;
}
.test1{
width: 100px;
height: 100px;
position: fixed;
top: 0;
left:0;
z-index: 100;
background: red;
}