We used to talk about PHP Framework, JavaScript Framework, but CSS Framework is not a common work in web application development, especially for web designer. The case of BluePrint CSS which aims to provide a ready to use CSS solution for developers and designers :

Blueprint is a CSS framework, which aims to cut down on your CSS development time. It gives you a solid CSS foundation to build your project on top of, with an easy-to-use grid, sensible typography, and even a stylesheet for printing.

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The advantage of having a CSS framework is certainly to make work with stylesheets more easy, in addition to providing a flexible way to design the page layout, and make it more accessible and easy to print. I’ll definitely go for such frameworks, rather than writing myself all css on the fly.