This simple calculator app uses HTML, CSS and JavaScript. This project is great for a beginner to learn basic web design.
I typically start off my students with notepad and work them up to this level before switching to fancy editors and IDEs. I've created this primarily for my students to play around with. Next is to have them recreate this using more modern standards like Twitter's Bootstrap, and jQuery or event listeners in JavaScript.
It's got a cute digital font you can pull off of https://www.dafont.com/digital-7.font. The CSS uses a gradient which makes the buttons look pretty.
You can download this and use it as you see fit.