z-index为什么不起作用？

[your_baobei]

<div id="tip" style="border-style: outset; border-width: 0px 2px 2px 0px; border-color: #C0C0C0; position:relative;z-index:500;display:none; background-color: #FFFFFF; width:390px; "></div>
       <div style="z-index:-1" class="left cntList border">

[大漠苍狼woft]

<div id="tip" style="border-style: outset; border-width: 0px 2px 2px 0px; border-color: #C0C0C0; position:relative;z-index:500;display:none; background-color: #FFFFFF; width:390px; "></div>
<div style="z-index:-1;position:relative" class="left cntList border">

[IE11下面经常卡到爆]

<div id="tip" style="min-height:200px;...">

[dalmeeme]

需要同时设置position:absolute;否则z-index是不起作用的。