浮动，清除浮动
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Document浮动</title>
</head>
<style type="text/css">
.pent{
width: 50px;height: 80px;background: red;float: left;
}
.mat{
width: 60px;height: 30px; float: left;background: #0000FF;
}
.fen{
width: 130px; height: 20px;background: #000000;
}
.cent{
clear: both;
}
</style>
<body>
<div>1</div>
<div>2</div>
<div class="fen cent">3</div>
</body>
</html>