There are a lot of open source CSS Framework. Most designers would have heard of the term ‘CSS Frameworks’, for those who don’t know, from here they can get brief description: Wikipedia:
Main features of good CSS Framework is to 1) rapidly speed up our development time, 2) should have a very small size, 3) have good documentation and tutorials and 4) have clean grid structure. You will need a basic understanding of the CSS framework you are going to use to understand why and how things get solved.
I am going to show you 14 promising CSS Framework that worth a look.