I'm having this same problem to. With no widths specified, it looks great in IE7, IE8, Firefox, and Chrome. IE6 is the only one that isn't working. If I do as Bend did and set a fixed width for everything, it renders in IE6 but that doesn't really work for me as some of my menu items are 4 characters long ("Home") and some are as many as 20 characters long.
PLEASE HELP!! This site has to go live this Friday and the IE6 menu issue is the only thing holding us up!
Here is my CSS. Right now I have a IE6 CSS that is included conditionally that sets a fixed width, but again I'd rather not do fixed with if at all possible.
1 /*Position the entire menu (SET A FIXED WIDTH IN IE6MENU.CSS!)*/
2 .mnuMain{position:absolute; top:94px; font:normal normal bold 16px Arial;}
3 /* Tier 1 UL, this is the container for the entire menu - Set width to 0px to make the menu vertical */
4 .mnuMain ul.AspNet-Menu{width:724px; z-index:255;}
5 /* Tier 1 LI - This is each top level item. Width should be preceding width / # of top level items */
6 .mnuMain li{height:48px !important; text-align:center; z-index:255;}
7 /* Tier 2 UL */
8 .mnuMain ul.AspNet-Menu ul{width:200px; top:100%; left:0; background:#7f1f11; z-index:255; border:solid 1px white; border-top:none;}
9 /* Tier 2+ LI - Make this the same width as the preceding */
10 .mnuMain li li{width:200px; z-index:255; margin:0; color:White; text-align:center; z-index:255; padding:3px 0px;}
11 /* Tier 3+ UL */
12 .mnuMain ul.AspNet-Menu ul ul{width:200px; top:5px; left:200px; z-index:255; border:solid 1px white;}
13 /* BACKGROUND of items being hovered over */
14 .mnuMain li:hover,
15 .mnuMain li.AspNet-Menu-Hover{background:#c9c099; z-index:255;}
16 /* Tier 1 As and spans */
17 .mnuMain a,
18 .mnuMain span{color:White; z-index:255; padding:14px 7px;}
19 /* Tier 2+ As and spans (if a different size from tier 1) */
20 .mnuMain li li a,
21 .mnuMain li li span{color:White; z-index:255; padding:3px 0px;}
22 /* Leaves */
23 .mnuMain li.AspNet-Menu-Leaf a,
24 .mnuMain li.AspNet-Menu-Leaf span{background-image:none !important; z-index:255;}
25 /* TEXT of items being hovered over */
26 .mnuMain li:hover a,
27 .mnuMain li:hover span,
28 .mnuMain li.AspNet-Menu-Hover a,
29 .mnuMain li.AspNet-Menu-Hover span,
30 .mnuMain li:hover li:hover a,
31 .mnuMain li:hover li:hover span,
32 .mnuMain li.AspNet-Menu-Hover li.AspNet-Menu-Hover a,
33 .mnuMain li.AspNet-Menu-Hover li.AspNet-Menu-Hover span,
34 .mnuMain li:hover li:hover li:hover a,
35 .mnuMain li:hover li:hover li:hover span,
36 .mnuMain li.AspNet-Menu-Hover li.AspNet-Menu-Hover li.AspNet-Menu-Hover a,
37 .mnuMain li.AspNet-Menu-Hover li.AspNet-Menu-Hover li.AspNet-Menu-Hover span
38 {color:#7f1f11;}
39 /* The tier above this one is hovered */
40 .mnuMain li:hover li a,
41 .mnuMain li:hover li span,
42 .mnuMain li.AspNet-Menu-Hover li a,
43 .mnuMain li.AspNet-Menu-Hover li span,
44 .mnuMain li:hover li:hover li a,
45 .mnuMain li:hover li:hover li span,
46 .mnuMain li.AspNet-Menu-Hover li.AspNet-Menu-Hover li a,
47 .mnuMain li.AspNet-Menu-Hover li.AspNet-Menu-Hover li span
48 {color:White;}
49 /* This tier is selected */
50 .mnuMain .AspNet-Menu-Selected{color:#7f1f11; background-color:#c9c099;}
51 /* A tier below this one is selected */
52 .mnuMain .AspNet-Menu-ChildSelected{color:#7f1f11; background-color:#c9c099;}
53 /* A tier above this one is selected */
54 .mnuMain .AspNet-Menu-ParentSelected{color:White;}