Created and designed (in browser) a pure HTML and CSS expandable accordion for fun. I tried to think outside of the box when designing this (no pun intended).
I used a (hidden) checkbox to toggle the expanding and collapsing of each panel. I am not fully satisfied with the animations, but I will revisit that at a later time.
Caveat: Each section opens and closes independently of each other. Created for desktop only—I haven't checked mobile.