<!DOCTYPE html>
<html>
<head>
<style>
div.container
{
width:30em;
border:1em solid;
}
div.box
box-sizing:border-box;
-moz-box-sizing:border-box; /* Firefox */
-webkit-box-sizing:border-box; /* Safari */
width:50%;
border:1em solid red;
float:left;
</style>
</head>
<body>
<div class="container">
<div class="box">这个 div 占据左半部分。</div>
<div class="box">这个 div 占据右半部分。</div>
</div>
</body>
</html>