1.19 浮动
元素左右浮动，根据浏览器大小而重新排布；
float 开始浮动
<head>
<style>
div {
float: left;
width: 100px;
height: 100px;
background-color: red;
margin: 20px;
}
</style>
</head>
<body>
<div>1</div>
<div>2</div>
<div>3</div>
<div>4</div>
<div>5</div>
clear 清除浮动
<head>
<style>
div {
float: left;
width: 100px;
height: 100px;
background-color: red;
margin: 20px;
}
</style>
</head>
<body>
<div>1</div>
<div>2</div>
<div>3</div>
<div>4</div>
<div>5</div>
<p>123</p>
<p style="clear: both">123</p>
<div>1</div>
<div>2</div>
<div>3</div>
<div>4</div>
<div>5</div>