leftExpansion

for a stack overflow example

HTML

<div id="page">
    <div id="leftCol">
        LEFT
    </div>
    <div id="container">
        <div id="rightCol">
            RIGHT
        </div>
    </div>
</div>

CSS

#page{
    background-color: purple;
}
#container {
    background-color: blue;
    width: 300px;
    height: 300px;
    margin: 0 auto;
}
#leftCol {
    float: left;
    background-color: red;
    height: 300px;
    min-width:100px;
    width:calc(100%/2 - 50px);
    max-width:200px;
}
#rightCol {
    margin-left:100px;
    background-color: green;
    height: 300px;
    width: 200px;
}