Menu ok in Firefox, fails in IE8

Not sure why the opening menu works in Firefox but fails in Internet Explorer.

In IE it is not centered, even though I’m using margin-left, margin-right: auto

.menu-vertical {
font-size: 1.5em;
font-weight: bold;
margin-left: auto;
margin-right: auto;
opacity: 0.8;
position: relative;
top: -450px;
width: 28% !important;
z-index: 99;

Do you have a doctype? If you do, does it have a URI at the end (a link)? If you answered no to either of these, get a proper doctype.