Is this sub menu positioning possible with CSS alone?

Apologies for the vague question, I can't think of a succinct, descriptive one.

I'm making a drop down menu out of unordered lists, which look a bit like this....

<ul id="menu">
    <li>Menu Item</li>
    <li>Menu Item
        <ul class="sub-menu">
            <li>Sub Menu Item</li>        
            <li>Sub Menu Item</li>
        </ul>
    </li>
    <li>Menu Item</li>
</ul>

However, I'm wondering wether this sort of positioning behaviour would be possible (Images explain this better I think)

Case 1

Sub menu is centred to it's containing list item, but is wider than it's containing list-item.

Case 2

Sub menu should be centered, but there's not enough left-space in the root list, so it keeps flush against the root lists left edge.

Case 3

Like Case 2, but this time it's the right edge.

I think it can be broken down into two tricky CSS problems.

1. How to center a sub menu to it's container, despite being wider than it's container.
2. How to prevent the submenu going beyond the bounds of the root-list element.

I'll be implementing this behaviour in Javascript since I'm on a deadline. But for future reference, it would be nice to kn开发者_如何学运维ow if this could be accomplished with CSS alone.

There's a JSFiddle here to play with if you have any theories.

http://jsfiddle.net/AxnVU/28/

You now have a div wrapping your submenus, so you can use some tricks to position the submenus using the text-align property.

If you don't want the gray background of the submenu, you can remove that and add a background color only to the ul inside the div.submenu, to have the same effect of your pics.

The idea is: you have a div occupying all the avaiable space (the width of your big menu), which contains the submenu. The submenu, displayed inline, can be manipulated with the text-align property to stay at the left, center or right of the avaiable space.

Edit

http://jsfiddle.net/AxnVU/69/

Here is another option with more menu voices, since the previous only worked for three voices. This time we need to specify a submenu wide three times a menu voice (but could be more also), so 300% wide, and center it by giving negative left margin (-100% in this case, whih is one menu voice). Inside the submenu there is the list item horizontal center with the same trick as before.