清除浮动

<div class="clear"></div>

.clear {
    clear: both;
}

<div class="container clearfix">
 <div class="div1">333</div>
 <div class="div1">333</div>
 <div class="div1">333</div>
</div>
<div class="div2">9999</div>

.clearfix:after {
    content: ".";
    display: block;
    height: 0;
    visibility: hidden;
    clear: both;
}
.clearfix {
    zoom: 1;  /* 触发hasLayout 兼容IE 6、7 */
}

<div class="container">
    <div class="div1">333</div>
    <div class="div1">333</div>
    <div class="div1">333</div>
</div>
<div class="div2">9999</div>

.container {
    width: 500px;
    overflow: hidden;
    zoom: 1;
}
.div1 {
    float: left;
    width: 100px;
    height:100px;
}
.div2 {
    width: 500px;
    height: 200px;
}