利用::after处理元素塌陷问题

<style>

#nav li{

list-style: none;

float: left;

}

.clearfix::after{

content: '';

display: block;

clear: both;

}

<ul id="nav" class="clearfix">

<li>01</li>

<li>02</li>

<li>03</li>

</ul>

</style>