Oh, sorry you won't see the diagonals unless the top/bottom borders are diferent colors to the left/right ones. The rounding I was talking about is mathematical rounding. As it can't display a diagonal with a 1px border it's all or nothing.
Yes, except it doesn't work in IE, check out
http://www.meyerweb.com/eric/css/edge/menus/demo.html
Works in mozilla firefox opera and just about anything but IE. Shame IE is used by 90% of the people.....
However, I have just thought of an Idea. You could take one of those menus and replace the :hover with a class. Then just have onmouseover="this.className='blah';"
I think that would be the most lightweight cross-browser multi-level dynamic html menu ever! Plus you could include the hover so it works in moz with javascript off. Thats a pretty cool project that I think I'm gonna do saturday morning!! Wooo
I still really don't know about the top border in IE, you could try on the csscreator.com forums, but it's so specialised there that it takes a while to get any answers.
//edit: so I got bored and made the menu. The JS is pretty lightweight but I had to do ome faffing to get it to work on IE, and I'm convinced that it will not work on Opera. But I haven't tested.
http://www.caffeinefuelled.net/tests/menu.html
Obviously needs some formatting doing to it

but talk about efficient 0.5k javascript for a multi level menu

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