The construction is a simple nested set of lists, and is completely controlled and styled via the CSS. Scripting is used only for the purpose of making IE/Win obey the :hover rules on non-links.
This method works very well in IE5 through IE6, all fairly recent Gecko browsers, Opera 7, and Safari, but not in Explorer for the Mac, which fails to support the basic popup method. IE5/Mac may be made to show the entire nested list set via a special targeted rule, so that those users can access all the links.