For all the latest browsers, operating systems, PC, tablets and smartphones. With :hover action fallback for IE7 and IE8;

Click/Tap the top level list items to open the droplist, then either click/tap another top level list item or click/tap the same list item to close that open droplist.

For tablets and smartphones click/tap the menu icon '☰', at the top right of the page, to slide out the top level list items and click/tap a top level item to slide down the sub menus.

Click/tap anywhere outside of the slide out menu to close all open sub menus and slide back the menu panel.