Edit in JSFiddle

<div blass="box">
    <p class="left">我设置了左浮动 float: left, 但是我的父元素没有触发 BFC</p>    
</div>
<div class="clear"></div>
<div class="box bfc">
    <p class="left">我设置了左浮动 float: left, 而且我的父元素触发了 BFC</p>
</div>

.box{
    border: 1px solid #9a9a9a;
}
p{
    background-color:gold;
}
.left{
    float:left;
}
.bfc{
    overflow:hidden;
    .box{
    border: 1px solid #9a9a9a;
}
p{
    background-color:gold;
}
.left{
    float:left;
}
.bfc{
    overflow:hidden;
    *zoom:1;
}
.clear{
    clear:both;
}
}
.clear{
    clear:both;
}