<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
*{margin: 0;padding: 0;}
.box1{
width: 100px;
height: 300px;
background: red;
position:absolute;
top:100px;
left:100px;
z-index:3;
}
.box2{
width: 300px;
height: 100px;
background: blue;
position:relative;
top:150px;
left:10px;
z-index:4;
}
.btn1{
width: 300px;
height: 100px;
background: red;
position:relative;
top:100px;
left:100px;
}
.btn2{
width: 100px;
height: 300px;
background: pink;
position:absolute;
top:-40px;
left:30px;
z-index:-3;
}
</style>
</head>
<body>
<div class="btn1">
<div class="btn2"></div>
</div>
</body>
</html>