Here's some information I found from the site:
What are Design Principles?
Design Principles are a set of considerations that form the basis of any good product.
Why use them?
Design Principles help teams with decision making. A few simple principles or constructive questions will guide your team towards making appropriate decisions.
Who uses them?
Organisations, individuals and product teams have benefited from writing and following their principles.
The aim of the site is to help us analyse what good Design Principles are. How Design Principles are created and measured. How they develop. Design Principles should be improved over time, how can we build a history of Design Principles and their improvements?
There are still a few more things to come which are listed on the roadmap.
Personally, I do think the creators did a good job with their web design. I'm enjoying clicking around on desktop.
What I think a lot is missing is that 80% of the benefits actually are found within the process itself. This is because the team gets to reflect, discuss, and show their own thinking.
We had 6 sessions spread out over a couple of week with different exercises and it was time well spent, even disregarding the actual outcome (4 design principles).