In initial state menu is horizontal, but it swithes to vertical mode when the menu is open.
This idea occurred to me initially when I was thinking about the most compact ways for a menu to be represented and how many menus these days tend to have multiple columns in them. Turning the menu vertical seemed to be somewhat obvious way to gain more vertical space for columns, and as an added bonus I didn’t need to hack :target so that menu items can be changed.
Now I wonder if this kind of menu has been done before? I’ve seen a lot of them but I haven’t really seen any other to switch from horizontal to vertical (or doing the opposite). Whether this is usable on a real site… I guess isn’t up to me to say.