HTML & JS Interaction Help

I’m working through a focused exercise in frontend development and have reached the point where the interactive pieces have to come alive. The structure and basic styling are already in place, but I need someone comfortable with plain HTML, vanilla JavaScript and modern browser APIs to wire up the dynamic behaviour.

Your job is to take the existing markup I will share, add or modify the script so the required interactions perform smoothly, and ensure the result feels polished in a real-world environment (no frameworks are involved here—just clean, readable code).

Once finished, please return the updated HTML/JS files with concise inline comments so I can follow the logic. A brief note outlining any key decisions or edge-case considerations will be appreciated as well.

If you enjoy tackling self-contained frontend puzzles and have an eye for seamless user interaction, this should be a quick, satisfying gig.