为什么Div外面加一个Table的时候就取不到他的offsetHeight 急


<body>
<table >
<tr>
<td>       
<div style="margin-top: 5px; margin-bottom: 3px">
    <img src="/Images/cat_arrow.gif" align="absmiddle" alt="" />
    按<b>款式</b>选择：<span id="_spnItem" style="display: none; padding-left: 15px; background: url(/images/category_view_all.gif) rgb(255,255,255) no-repeat 0% 0%; color: rgb(34,34,34); cursor: pointer;" onclick="ShowHideItem(this)">显示所有</span>
</div>
<div id="_divItem" style="work-break: break-all; overflow: hidden;">
    <table id="UC_MenuItem1_rptList" cellspacing="0" border="0" style="border-collapse:collapse;margin-left: 20px">
    <tr>
        <td style="width:130px;">
            <a href='/ListAll.htm?&ItemCode=0002'>网球鞋</a> <span class="cat_num">(2)</span>
        </td><td style="width:130px;">
            <a href='/ListAll.htm?&ItemCode=0015'>篮球鞋</a> <span class="cat_num">(6)</span>
        </td><td style="width:130px;">
            <a href='/ListAll.htm?&ItemCode=0020'>足球鞋</a> <span class="cat_num">(1)</span>
        </td><td style="width:130px;">
            <a href='/ListAll.htm?&ItemCode=0026'>跑步鞋</a> <span class="cat_num">(15)</span>
        </td>
    </tr>
</table>
</div>
<table border="0" width="100%">
    <tr>
        <td colspan="99" style="height: 5; background: url(/images/dot_line.gif) repeat-x left bottom;">
        </td>
    </tr>
</table>
</td></tr></table><!-- 放到这里 -->
<script type="text/javascript">
 var _divItem = document.getElementById("_divItem");
 alert(_divItem.offsetHeight);
 if(_divItem.offsetHeight > 30)
 {
    document.getElementById("_spnItem").style.display="";
    _divItem.style.height="22px";
 }
</script>
</body>