Web Basic Arithmetic Calculator -- 2

I need a lightweight web-based calculator that handles the four core arithmetic operations—addition, subtraction, multiplication, and division—with clean, intuitive styling and instant results. Everything should run client-side in modern browsers; plain HTML, CSS, and vanilla JavaScript are fine, but you’re welcome to use a minimal framework if it speeds things up without bloating load times.

Please make sure:

• The interface is responsive so it looks good on both desktop and mobile browsers.
• Input validation prevents NaN or divide-by-zero errors.
• The code is well-commented and easy for me to extend later (e.g., to add a percentage button).

Deliverables: a zip or Git repo containing the source files and a brief README explaining setup and any build steps (if you chose a framework). Once I can open index.html and start calculating right away, the job is complete.