Align a div center of page vertically and horizontally

Today i need to align a div center in page horizontal as well as verticle.


css :
body
{
font-size: 14px;
font-family: verdana, helvetica, sans-serif;
color: #333;
}
p
{
margin: 2em;
}

#centerdiv
{
position: absolute;
top: 50%;
left: 50%;
width: 50em;
height: 26em;
margin-top: -13em; /*set to a negative number 1/2 of your height*/
margin-left: -25em; /*set to a negative number 1/2 of your width*/
border: 1px solid #ccc;
background-color: #f3f3f3;
}


html is :