导航菜单无法在IE版本中使用

Question

我有一个网站：http://www.icontactlens.com.sg

当我点击导航菜单“关于我们”时，它会在Chrome和Firefox上显示子菜单。但是，当我点击IE（8,9,10版本）时，它无法正常工作。

这是CSS代码：

/* main navigation bar */
#navigation{ 
    padding:0 0 30px 0;
}

/* first level menu item's font face */
#navigation > ul > li > a{
    font-family: 'Oswald', arial, serif; 
}

/* first level links */
#navigation, #navigation a{ 
    color:#fff;
    text-decoration:none;
    font-size:14px;
    line-height:100%;
    display:block;
}

/* inacvtive - expandable items */
#navigation a.inactive{ 
    background:url(../images/plusminus.png) right 3px no-repeat;
}

/* acvtive - expanded items */
#navigation a.active{ 
    background:url(../images/plusminus.png) right -12px no-repeat;
}   

/* current page items and hover states */
#navigation a:hover, #navigation ul li.current_page_item a, #navigation ul li.current-menu-ancestor a{  
    color:#9D9D9D;
}       

/* current page items and hover states */
#navigation ul li.current_page_item a:hover, #navigation ul li.current-menu-ancestor a:hover{  
    color:#fff;
}

#navigation ul ul li.current-menu-item a{
    color:#D0D0D0;
}


#navigation ul{
    padding:1px 0 0 0;
    display:block;
    margin:0;
    list-style:none !important;
}

/* First Level */
#navigation > ul{
    background:url(../images/transparent-pixel.png) top repeat-x;
}

#navigation > ul > li{
    background:url(../images/transparent-pixel.png) bottom repeat-x;
    padding:10px 0 10px 0;
}

#navigation ul li{
    display:block;
}

/* Sub Levels */
#navigation  ul ul{
    display:none;
    margin:10px 0 0 0;
}   

#navigation ul ul li{
    margin:0 0 0 0;
    padding:5px 0 5px 0;
}

#navigation ul ul li a{
    font-size:12px;
    color:#C4C4C4;
}

/* Third Level */
#navigation  ul ul ul{
    margin:10px 0 0 0;
    background:url(../images/transparent-pixel.png) repeat-y;
}   

#navigation ul ul ul li{
    margin:0 0 0 10px;
}

我在CSS样式表上遗漏了什么吗？