The purpose of CSS Zen Garden is to showcase CSS and what is possible through the use of it. This design is my favorite because it's clear, concise, and easy to navigate. The color scheme is pleasing and it's simple.
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I would rate it a 10. I don't see any issues with it.
The site is very user friendly I'd give it an 8 only because I had to scroll about half a screen to read what the site is about. It's a design showcase. If you follow the links for the CSS examples on the sidebar, the kinda plain main page is transformed into stunning design examples of how CSS can transform your site.
The purpose of the site is to show the possiblities of CSS and I think it does an excellent job of doing that. I'm sold, and want to know all there is to know about CSS