清除浮动方法。

   .content {
            overflow:hidden;
            width:100%;
            height:200px;
            border:2px dotted blue;
        }
            .content .left {
                float:left;
                width:200px;
                border:2px dotted blue;
                height:300px;
                background-image:url("1.jpg")
            }
            .content .right {
                margin-left:200px;
                border:2px solid green;
                height:400px;
                background-image:url("a.png")
            }
        .buttom {
            clear:both;
            height:200px;
            width:100%;
            border:1px solid red;
        }

  <div class="content">
            <div class="left"></div>
            <div class="right"></div>
        </div>
        <div class="buttom"></div>

<div id="layout">
  <div id=‘left‘>Left</div>
  <div id="right">Right</div>
  <div style="clear:both"></div>
</div>

 #layout:after{
            content:".";
            display:block;
            clear:both;
            visibility:hidden;
            height:0px;
        }