Why I am getting Element offsetHeight "0"? even element original height is not showing

```
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
<style>
/* Added for Menu    */
.navbar
{
  list-style: none;`enter code here`
  padding: 0 10px;
  margin: 0;
  float: left;
  width: 980px;
  background: url(images/navbar.gif) repeat-x;
}
.navbar ul
{
 list-style: none;
  padding: 0px;
  margin: 0;
  float: left;
  width: 260px;
}
.navbar ul li
{
    float:left;
    position:relative;
    z-index:1000;
    margin:0px;
    padding:0px;
}
.navbar ul li ul
{
    display:none;
    border:none;
    width: 260px;
    padding:0 0px 0px 0px;
}
.navbar ul li a
{
     font-size: 12px;
     float:left;
     display:block;
     line-height:3.1em;
     padding:0 16px 0 16px;
     text-decoration:none;
     color:#FFF;
}
.navbar ul li:hover
{
    width:auto;
}
.navbar ul li:hover a
{
    background:url(images/navbar_subnav.png)  repeat-x scroll 0 0 transparent;          text-decoration:none;
}
.navbar ul li:hover ul
{
    display:block;
    position:absolute;
    z-index:998;
    top:0px;
    margin-top:25px;
    left:0;
}
.navbar ul li:hover ul li ul
{
    background-position: center bottombottom;
    cursor: pointer;
}
.navbar ul li:hover ul li a
{
    display:block;
    line-height:1.3em;
    padding:4px 16px 4px 16px;
    border-bottom:1px solid #205284;
    border-left:1px solid #205284;
    border-right:1px solid #205284;
    border-top:1px solid #205284;
    z-index: 10000;
    width:200px;
  }
.navbar ul li:hover ul li a:hover
{
     background-color:#000;
     text-decoration:underline;
     color: #0099FF;
     cursor: hand;

}
.navbar ul li a:hover ul
{
    display:block;
    position:absolute;
    z-index: 998;
    top:3.1em;
    t\op:3.0em;
    left:0;
    marg\in-top:0.1em;
}
.navbar ul li a:hover ul li a
 {
    display:block;
    height:1px;
    line-height:1.3em;
    padding:4px 16px 4px 16px;
    background-color:#000;
    font-weight:normal;
    color:#FFF;
}
.navbar ul li a:hover ul li a ul
{
    visibility:hidden;
    height:0px;
    width:0px;
    position:absolute;
    z-index: 997;
}
.navbar ul li a:hover ul li a:hover
{
    background-color:#000;
    text-decoration:underline;
}

</style>
</head>

<body>
<div class="navbar">
<ul>
            <li>
                <a href="#">menu item 1</a>
                <ul style="top:0px" >
                    <li style="width:288px;"><center><a href="#" onmouseover="movedown()" onmouseout="stopscroll()"><img src="images/menu-arrow-down.png" width="8" height="6"   alt="Down"></a></center></li>
<script> //<![CDATA[
var speed=2
iens6=document.all||document.getElementById
ns4=document.layers
if (iens6){
document.write('<div id="container" style="position:relative;width:310px;height:165px;overflow:hidden;border:0px solid #205284">')
document.write('<div id="content" style="position:absolute;width:290px;left:0px;top:-1px;">')
}
//]]></script>
<ilayer name="nscontainer" width="190">
<layer name="nscontent" width="290" height="155" visibility="hidden">
                    <li style="top:-1px;"><a href="#">sub menu item 1  menu item 1</a></li>
                    <li style="top:-1px;"><a href="#">sub menu item 2  menu item 1</a></li>                    <li style="top:-1px;"><a href="#">sub menu item 3</a></li>
                    <li style="top:-1px;"><a href="#">sub menu item 1</a></li>
                    <li style="top:-1px;"><a href="#">sub menu item 1</a></li>
                    <li style="top:-1px;"><a href="#">sub menu item 2</a></li>              
                    <li style="top:-1px;"><a href="#">sub menu item 3</a></li>
                    <li style="top:-1px;"><a href="#">sub menu item 1</a></li>
                    <li style="top:-1px;"><a href="#">sub menu </a></li>
                    <li style="top:-1px;"><a href="#">sub menu </a></li>
                    <li style="top:-1px;"><a href="#">sub menu </a></li>
                    <li style="top:-1px;"><a href="#">sub menu </a></li>              
                    <li style="top:-1px;"><a href="#">sub menu </a></li>
                    <li style="top:-1px;"><a href="#">sub menu</a></li>

</layer>
</ilayer>
<script> //<![CDATA[
if (iens6)
{
document.write('</div></div>')
var crossobj=document.getElementById? document.getElementById('content') : document.all.content
//alert("document.getElementById('content').offsetHeight :"+ document.getElementById('content').offsetHeight);
alert("crossobj.prototype.outerHeight" + crossobj.prototype.outerHeight);
var contentheight=crossobj.offsetHeight
//var contentheight=document.getElementById('content').style.height;
//alert("ContentHeight :"+contentheight);
}
else if (ns4)
{
    var crossobj=document.nscontainer.document.nscontent
    var contentheight=crossobj.clip.height
}
function movedown(){
if (window.moveupvar) clearTimeout(moveupvar)
{
    if (iens6&&parseInt(crossobj.style.top)>=(contentheight*(-1)+100))
    {
        crossobj.style.top=parseInt(crossobj.style.top)-speed+"px"
    }
    else if(ns4&&crossobj.top>=(contentheight*(-1)+100))
    {
        crossobj.top-=speed
    }
    /*else
    {
        crossobj.style.top=parseInt(crossobj.style.top)-speed+"px"
    }*/
}
    movedownvar=setTimeout("movedown()",20)
}
function moveup()
{
    if (window.movedownvar) clearTimeout(movedownvar)
    {
        if (iens6&&parseInt(crossobj.style.top)<=0)
        {
        crossobj.style.top=parseInt(crossobj.style.top)+speed+"px"
        }
        else if (ns4&&crossobj.top<=0)
        {
        crossobj.top+=speed
        }
        moveupvar=setTimeout("moveup()",20)
    }
}
function getcontent_height()
{
    if (iens6)
    {
        //alert("getcontent_height : "+document.getElementById("content").style.height);
        //contentheight=document.getElementById("content").style.height;
        contentheight=crossobj.offsetHeight
    }
    else if (ns4)
    {
        document.nscontainer.document.nscontent.visibility="show"
    }
}
window.onload=getcontent_height;
//]]></script>
                    <li><center><a href="#" onmouseover="moveup()" onmouseout="stopscroll()"><img src="images/menu-arrow-down.png" width="8" height="6" alt="Down"></a> </center></li>
                </ul>
            </li>
           </ul> 
      </div>    
</body>
</html>

```