SOURCE

<div class="father clearfix">
    <div class="child"></div>
    <div class="child"></div>
    <div class="child"></div>
    <div class="child"></div>
</div>

.father{
    width: 100%;
    border: 1px solid #ff9900;
}

.child{ 
    width: 100px;
    height: 100px;
    background: pink;
    float: left;
}
.clear{
    clear: both;
}

.clearfix:before,.clearfix:after{
    content: "";
    display:table;
}
.clearfix:after{
    clear: both;
}
.clearfix{
    *zoom:1;
}