Refactor using HTML, CSS, and JS

This is the second challenge I've refactored. I focused on trying to identify which elements could be looked at as possible reusable components and separated out my CSS accordingly. Whether this is the right approach or not only time will tell.

Also added a JS utility to ensure only the selected item is displayed (inspired by Maria's solution. The bouncing box was inspired by ApplePieGiraffe's solution. Thank you both.

The graphics still gave me fits but at least I had a better idea of why things were going wrong. Also not convinced I've found the most useful way of dealing with them. Any and all suggestions for improvement are welcome.