Edit in JSFiddle

<div class="container">
    <div class="left">左浮动</div>
    <div class="right">右浮动</div>
</div>

.container{
    border:1px solid blue;
    overflow:auto;/*我就可以清除浮动*/
}
.left{
    float:left;
    width: 50px;
    height: 50px;
    background-color:yellow;
}
.right{
    float:right;
    width: 100px;
    height: 100px;
    background-color:red;
}