Edit in JSFiddle

<div class="box gold left">
    <p>我设置了float:left;width:200px;</p>
</div>

<div class="box blue bfc">
    <p>我没有设置浮动，触发了BFC,width:300px;overflow:hidden;*zoom:1</p>
</div>

.box{
    border:1px solid #9a9a9a;
}
.gold{
    background-color:gold; 
}
.blue{
   background-color:lightblue;  
}
.bfc{
    width:300px;
    overflow:hidden;
    *zoom:1;
}
.left{
    float:left;
    width:200px;
}
.nobfc{
    width:300px;
    border:
}