Main Reference
Responsive Multi Level Dropdown Menu with CSS | CSS3 Animated Hamburger Menu
It can decide whether left-align or right align when it reaches to right side of the window. i.e., the sub menu will not go overboard.
The best part of this reference is that you can get the source code from CodePen.
Source Code: https://codepen.io/smashtheshell/pen/…
Other references
Multi-Level Dropdown Menu using Html Css & Javascript | Responsive
multiple level nav menu
Responsive Dropdown Menu Bar Using HTML CSS & JavaScript
multiple level nav menu with icon to the left (not right aligned)
on mobile mode, the dropwodn is full screen
Responsive Dropdown Menu Bar With HTML CSS And JAVASCRIPT
multiple level nav menu