百度登陆增加一个背景DIV，为什么要写iframe

百度登陆增加一个背景DIV，为什么要写iframe

<div id="TANGRAM__PSP_3__" style="top: 0px; left: 0px; position: fixed; width: 1920px; height: 662px; opacity: 0.15; z-index: 60000; background-color: rgb(0, 0, 0); display: none;"><iframe frameborder="0" style="filter:progid:DXImageTransform.Microsoft.Alpha(opacity:0);position:absolute;top:0px;left:0px;width:100%;height:100%;z-index:-1" src="about:blank"></iframe><div style="position:absolute;top:0px;left:0px;width:100%;height:100%;z-index:-1;">&nbsp;</div></div>

下面这样写没发现有BUG,能遮住select呀?

    <div style="top: 0px; left: 0px; position: fixed; width: 1920px; height: 662px; opacity: 0.15; z-index: 60000; background-color: rgb(0, 0, 0);"></div>
    <form action="" method="post">
        <select>
            <option>6111</option>
            <option>5111</option>
            <option>4111</option>
            <option>3111</option>
            <option>2</option>
        </select>
        <input type="text" name="xxx"  placeholder="请填写用户名" autocomplete="off" />
        <input type="submit" name="submit" value="提交"   />
    </form>