Browser IE7+和opera中的z索引问题

以下是html：

<div id = "parent">
   <div id = "one" ></div>
   <div id = "two" ></div>
</div>

ie7+和opera中的问题是div one没有重叠div two，为什么会这样？

尝试使用以下方法：

<div id = "parent">
   <div class= "one" ></div>
   <div class= "two" ></div>
</div>
css:

#parent
{
  position:    relative;
width: 150px;
height: 150px;
background: yellow;
}
   #parent  .one
   {
     position:  absolute;
     z-index:   10;
     top:       50px;
     left:      50px;  
     width:     100px;
     height:    100px;
     background:  white;        
   }
   #parent .two
   {
     position:  absolute;
     z-index:   5;
     top:       50px;
     left:      50px;
     height:    50px;
     width:     50px;
     background: black;
   }

---

希望你能得到线索

将id更改为类与任何事情有什么关系？是否有效？如果未工作，则发送其输出的屏幕截图

<div id = "parent">
   <div class= "one" ></div>
   <div class= "two" ></div>
</div>
css:

#parent
{
  position:    relative;
width: 150px;
height: 150px;
background: yellow;
}
   #parent  .one
   {
     position:  absolute;
     z-index:   10;
     top:       50px;
     left:      50px;  
     width:     100px;
     height:    100px;
     background:  white;        
   }
   #parent .two
   {
     position:  absolute;
     z-index:   5;
     top:       50px;
     left:      50px;
     height:    50px;
     width:     50px;
     background: black;
   }