While I see I could say some more stuff in my commits, the commits themselves are not teaching material. At least, I don't have the time to do that, hence the simplicity of my commit messages.

You can check https://github.com/CSSLint/csslint/wiki/

https://github.com/CSSLint/csslint/wiki/Disallow-duplicate-properties
https://github.com/CSSLint/csslint/wiki/Require-standard-property-with-vendor-prefix

Those should answer most of your questions.