Almost every browser will understand the auto setting for a margin:
Code:
#div_id {
width: 760px;
margin: 0 auto;
}
If you absolutely feel you need to fully support IE 6, then this hack will work:
Code:
#div_id {
width: 760px;
left: 50%;
margin-left: -380px;
}
Frankly, I'd let IE6 slide these days. Neither CSS hacks nor the available "IE-updating" javascript libraries are worth the bother for niggling little details like horizontal positioning of containers. Your site will still work without the hacks, even if it isn't exactly the same in IE6 as it is in other browsers. And it just may give people a reason to get fundamentally fed up with a browser that has been obsolete for many years.