<!DOCTYPE html>
<html>
<head>
<style>
div.container {
width: 30em;
border: 1em solid;
}
div.box {
box-sizing: border-box;
width: 50%;
border: 1em solid red;
float: left;
}
</style>
</head>
<body>
<div class="container">
<div class="box">This div occupies the left half.</div>
<div class="box">This div occupies the right half.</div>
<div style="clear:both;"></div>
</div>
</body>
</html>