<!DOCTYPE html>
<style>
#outer {
width: 300px;
height: 300px;
background-color: blue;
position: relative;
}
#inner {
width: 100px;
height: 100px;
position: absolute;
left: 100px;
top: 100px;
background-color: green;
}
</style>
<div id=outer>
<div id=inner></div>
</div>