<html>
<style>
#left
{
width: 100px;
height: 100%;
background: red;
float: left;
}
#right
{
margin-left: 100px;
background: green;
height: 100%;
}
</style>
<body>
<div id="left"> </div>
<div id="right"> </div>
</body>
</html>