Center div inside div (I know, its not working...)

Why does this not work? It aligns horizontally correct, but not vertically in Opera. In IE it dosent work at all.

http://img834.imageshack.us/img834/340/86238198.png

#footer
{
    position: absolute;
    top: 905px;
    width: 100%;
    min-width: 800px;
    height: 95px;
    margin: 0px;
    background-image: url('footerbg.png');
}

#center
{
    position: relative;
    wi开发者_运维百科dth: 20%;
    height: 70%;
    margin: auto;

    background-color: red;
}

In Transitional (quirks mode), IE maintains the behavior of its older browsers, so as to not break pre-existing websites that were constructed to look ok in IE 5. So in IE 6 and up, if you do not define a Strict doctype, then it will resort to its old incorrect behavior of not honoring margin:auto.