Div side by side without float

How can I make div 'left' and 'right' look like columns side by side?

I know I can use float:left on them and that will work... but on step 5 and 6 in here http://www.barelyfitz.com/screencast...s/positioning/ the guy says it is possible, I can't get it work though...

Code:

<style>
div.left {
    background:blue;
    height:200px;
    width:300px;
}

div.right{
    background:green;
    height:300px;
    width:100px;
}

.container{
    background:black;
    height:400px;
    width:450px;
}
</style>

<div class="container">
        <div class="left">
            LEFT
        </div>
        <div class="right">
            RIGHT
        </div>
</div>