BFC6

HTML

<div class="BFC">
    <div class="box">
         
    </div>
    <div class="box">
         
    </div>
</div>

CSS

.BFC {
        border: 5px solid #f00;
        width: 300px;
        overflow:hidden;
    }
 
    .box {
        border: 5px solid blue;
        width:100px;
        height: 100px;
        float: left;
    }