Q:

Keyboard friendly CSS menus

My Question is a sequel to this question

Keyboard accessible web dropdown menus?

While the above questions says that

We figured out how to show the menu
  with a keyboard shortcut, but I'm not
  sure how to select one of the entries

I have figured out how to select single menu items using accesskeys(and by underlining the key letter), but I do not know how to pop up a menu on keypress.
The menu is a XHTML/CSS only menu, XHTML being
<ul>
    <li>Menu 1
        <ul>
            <li>Item 1</li>
            <li>Item 2</li>
            <li>Item 3</li>
        </ul>
    </li>
    <li>Menu 2
        <ul>
            <li>Item 1</li>
            <li>Item 2</li>
            <li>Item 3</li>
        </ul>
    </li>
</ul>

A:

You can't.
Using accesskey will either activate or focus a link (depending on the browser). 
Once a link has focus, you could get the menu to show up with something like:
ul#mainMenu > li > a:focus + ul { display: block }

But then you wouldn't be able to interact with anything inside the menu, as it would vanish as soon as the focus moved away.
CSS is a nice tool for describing presentation — which is what it is designed for — it is a very poor tool for describing interaction logic. JavaScript is designed for that, so use the right tool for the job.
The least problematic drop down menu script I've come across is UDM4 but I'd usually try to avoid drop downs entirely.

