css.container{    margin: 30px auto;    width:600px;    height: 300px;  }.p{    border:solid 3px #a33;  }.c{    width: 100px;    height: 100px;    background-color: #060;    margin: 10px;    float: left;  } <div class="container">      <div class="p">            <div style="clear:both">//為什么不能寫元素之前？           <div class="c"></div>            <div class="c"></div>            <div class="c"></div>                                                                                                                        <div style="clear:both">        </div>      </div>帶有clear屬性的元素為什么不能寫浮動元素之前？