The 'main' branch is currently running on the Business Gateway new design prototype. This is currently being used by Early Adopters to integrate the new Submit an application to change the land register APIs (Send a document, submit an application, application information and notification).
This prototype used GOV.UK prototyping kit v. 13.16.2.
It is also uses the 'Side navigation' componenet from MOJ Design System
Design documentation is running on the 'design-documentation' branch.
Go to the GOV.UK Prototype Kit site to download the latest version and read the documentation.
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.
Read the project principles.