Go to the SMI Prototype site to see this in action.
The prototype has been upgraded from 5.3 to 8.7 by following the Updating the kit instructions. The code for the 5.3 application is in the app/v6 folder.
The Prototype Kit provides a simple way to make interactive prototypes that look like pages on GOV.UK. These prototypes can be used to show ideas to people you work with, and to do user research.