层固定在底部

<style type="text/css">       
        #bottomNav
        {
            background-color: red;
            z-index: 999;
            position: fixed;
            bottom: 0px;
            left: 0px;
            width: 100%;
            _position: absolute; /* for IE6 */
            _top: expression(documentElement.scrollTop + documentElement.clientHeight-this.offsetHeight); /* for IE6 */
            overflow: visible;
        }
    </style>
    <div id="bottomNav">
        漂浮层</div>