Currently the menu and it item submenus are divided between two different divs. It is not usual and convenient decision.
I propose some non significant changes that give us all submenus nested into they parent menuitem.
If this changes will be applied it will be possible to replace js menu processing with help of pure css in new skins/styles.
NB. unfortunately, with this changes the small triangle under selected parent menuitem in existing styles should be moved to left side of the menuitem.