<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.test{
display: inline-block;
}
.test div{
float: left;
}
</style>
</head>
<body>
<html>
<head>
</head>
<body>
<div style="width: 10px; height: 20px; background: red"></div>
<div style="width: 10px; height: 20px; background: red"></div>
<div style="width: 10px; height: 20px; background: red"></div>
<script>
var b = document.body;
b.classList.add('test')
alert(b.offsetWidth)
</script>
</body>
</html>
</body>
</html>