以两个div右上角对齐为例：
效果图：
1.同级定位
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>同级</title>
<style type="text/css">
body {
margin: 0;
padding: 0;
}
.main {
position: relative;
width:1000px;
height:600px;
background: gray;
}
.div_01 {
position: absolute;
top: 0;
right: 0;
width: 600px;
height: 400px;
z-index: 1;
background: yellow;
}
.div_02 {
position: absolute;
top: 0;
right: 0;
width: 200px;
height: 100px;
z-index: 2;
background: green;
}
</style>
</head>
<body>
<div class="main">
<div class="div_01">
div_01
</div>
<div class="div_02">
div_02
</div>
</div>
</body>
</html>
2.父子级定位
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>嵌套</title>
<style type="text/css">
body {
margin: 0;
padding: 0;
}
.div_01 {
position: relative;
width:600px;
height:400px;
background: yellow;
}
.div_02 {
position:absolute;
top: 0;
right: 0;
width:200px;
height:100px;
background: green;
}
</style>
</head>
<body>
<div class="div_01">
<div class="div_02">
div_02
</div>
</div>
</body>
</html>