Define a dimension for the parent container, e.g. div:
<div style="border: 2px solid red;position:relative;top:0%;left:0%;height:200px;width:300px">
<div style="border: 2px solid green;position:absolute;top:50%;left:50%;height:50%;width:50%">
</div>
</div>
Or
Another way is to just stretch the parent container, i.e. div, by its top, bottom, left, and right properties. Like this:
<div style="border: 2px solid red;position: absolute;top: 0px;bottom: 0px;left:0px;right:0px;">
<div style="border: 2px solid green;position:absolute;top:50%;left:50%;height:50%;width:50%">
</div>
</div>