Prototypes depict the future.
I've made them with HTML, Java, PowerPoint, and GUI layout tools.

For a conference with the theme, "Matching Solutions to Problems," I made a powerpoint prototype of an animation that showed exactly that. Click the icon at left, press F5, and sit back for a short show. A similar approach is at the heart of my simple conference timer.
With its array of GUI tools, HTML makes a fine tool for developing prototypes. The example was built for an experiment in web access. It is a client interface to the database of an imagined trucking company. The buttons all work; but data entered in fields is ignored.
Some interactive development environments have too many "property sheets." Since the goal is program text, I thought an augmented editor could use speech baloons to show the options at each point in the text. The icon here links to the Java prototype. Learning the deficiencies of the approach this way was much easier than building an entire editor.
As a go player, I wanted a go problem on my home page. And what better than a problem with my own initials! The icon will take you to my home page where you can click in the Java applet and solve the problem. I also have a page giving the fundamental go shape problems.