清楚浮动的两种方法:
方法一:给float标签的父级标签添加overflow: hidden/auto属性 例如:
<!DOCTYPE html>
<html>
<head>
<title>DIVCSS5实例 DIV与DIV覆盖</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<style>
.boxa{overflow: hidden;}
.boxa,.boxb{ margin:0 auto; width:400px;}
.boxa-l{ float:left; width:280px; height:80px; border:1px solid #F00}
.boxa-r{ float:right; width:100px; height:80px; border:1px solid #F00}
.boxb{ border:1px solid #000; height:40px; background:#999}
</style>
</head>
<body>
<div class="boxa"><!--方法一:给float标签的父级标签添加overflow: hidden属性-->
<div class="boxa-l">内容左</div>
<div class="boxa-r">内容右</div>
</div>
<div class="boxb">boxb盒子里的内容</div>
</body>
</html>
方法二:在float标签的父级标签闭合之前添加标签,使用clear:both清楚浮动
<!DOCTYPE html>
<html>
<head>
<title>DIVCSS5实例 DIV与DIV覆盖</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<style>
.boxa,.boxb{ margin:0 auto; width:400px;}
.boxa-l{ float:left; width:280px; height:80px; border:1px solid #F00}
.boxa-r{ float:right; width:100px; height:80px; border:1px solid #F00}
.boxb{ border:1px solid #000; height:40px; background:#999}
.clear{clear: both;}
</style>
</head>
<body>
<div class="boxa">
<div class="boxa-l">内容左</div>
<div class="boxa-r">内容右</div>
<div class="clear"></div><!--方法二:在float标签的父级标签闭合之前添加标签,使用clear:both清楚浮动-->
</div>
<div class="boxb">boxb盒子里的内容</div>
</body>
</html>
方法三:借助给父级元素添加after伪类
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<meta name="author" content="Chomo"/>
<link rel="start" href="http://www.14px.com" title="Home"/>
<title>利用box-sizing实现div仿框架</title>
<style type="text/css">
*{margin: 0;padding: 0;}
.parent{width: 300px; border: 1px solid;}
.son{width: 20px; height: 20px; background: #EE4063; float: left;}
span{float: right;}
.clear:after{
content: "."; /*添加句号*/
height: 0; /*让高度为0,否则会撑大父级框*/
visibility: hidden; /*让句号只站位不显示*/
display: block; /*一定要块级框显示,否则撑不开*/
clear: both;/*清楚浮动*/
}
</style>
</head>
<body>
<div class="parent clear">
<div class="son"></div>
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</div>
</body>
</html>