<html>
<head>
<style>
div.container {
width: 100%;
border: 2px solid black;
}
div.box {
box-sizing: border-box;
width: 50%;
border: 5px 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>