ituzebonheur7/ib ? reverse-engineered prompt

Build me a small browser based web app that feels complete and polished, using plain HTML, CSS, and JavaScript so I can just open it and use it without a complicated setup. Keep it lightweight and simple, but not bare bones. I want a clean single page experience with a modern look, responsive layout for phone and desktop, clear text and buttons, and enough interactivity that it feels useful instead of static.

Please make sensible product choices for anything that is not specified, and handle both the design and the behavior. Add friendly feedback when people click, submit, or make mistakes, plus basic validation and empty states so it feels thought through. Organize the code cleanly so it is easy to tweak later, and make the styling feel consistent with good spacing, readable typography, and smooth little transitions.

If you need inspiration for current browser friendly patterns, look up current docs online, but keep the final result simple and easy to run locally.