Pure CSS Vertical Multi-level Navigation Menu

Menu 1
- Submenu 1.1
  - Submenu 1.1.1
  - Submenu 1.1.2
    - Submenu 1.1.2.1
    - Submenu 1.1.2.2
- Submenu 1.2
Menu 2
- Submenu 2.1
- Submenu 2.2
Menu 3
- Submenu 3.1
- Submenu 3.2