[CSS] centering DIV only works in FF

programming web dev
1

I wrapped my whole site in this:


#wrapper {
	border: 1px solid black;
	width: 600px;
	margin: auto;
	background: #eff3f7;
}

It is supposed to center my site. Why does it only center in Firefox? It does not center in IE7 or IE6.

Page: http://joanna-levesque.org/main/bday.php
CSS: http://joanna-levesque.org/main/bday.css