I have following HTML
<div id="main">
    <div id="subdiv"> <span>
            Some Text
        </span>
    </div>
</div>
CSS
#main {
    height: 300px;
    border:2px solid green;
    width:100%;
   overflow:auto;
}
#subdiv {
    width:90%;
    height:90%;
    border: 2px solid red;
}
Now when the browser window size is reduced i want the subdiv to remain always it's original size i.e 90% and just add scrollbar to main div, but no matter what i try the subdiv is also getting reduced. 
Like in this JSFIDDLE, want the div having red margin to remain constant and add scroll bar to div having green border.