leftExpansion

for a stack overflow example

HTML

<div id="page">
    <div id="container">
        <div id="leftCol">
            LEFT
        </div>
        <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;
    width: 100px;
}
#rightCol {
    float: left;
    background-color: green;
    height: 300px;
    width: 200px;
}