Check the @2x.
Mobile menu I did for a new project, mostly CSS3, some JS. The idea is to mimic the native app experience in the browser by "pushing" the website in the background and bringing the navigation to the front. Runs much smoother then in the gif.
The "plus" is just an indicator done with a pseudo element that the menu item has children, when opened the parent is cloned inside as the first element in case it contains a link. If the parent URL is # then it's not cloned inside.