Comment by Theoleff

Less than you'd think. A lot of it is just knowing the right element exists. A slide-out menu that closes on Esc or when you click outside is a button with popovertarget, a nav with popover, and a few lines of CSS for the animation.

(I'm biased, I ended up writing a book about this kind of stuff: https://theosoti.com/you-dont-need-js/)