Proton

Menus usually comprise an unordered list (ul) wrapped in a nav element belonging to the menu class. Use the menu-bar modifyer class for a horizontal menu.

Sub-menus are nested lists. In the case of horizontal menu bars, nested sub-menus appear as pure-CSS drop-downs. Proton only supports one-level sub-menus in horizontal navigation. If you really need multi-tiered flyout menus in a prototype, your navigation is perhaps too complex already and you should reconsider your site's architecture.