http://www.w3cplus.com/css/css-architectures-principles-of-code-cleanup.html

 

http://www.w3cplus.com/content/css3-multiple-backgrounds

 

http://www.w3cplus.com/source/front-end-developer-excellent-tool.html

 

http://www.w3cplus.com/css/css-specificity-things-you-should-know.html

 

http://css3clickchart.com/#multiple-columns