Compass

http://www.sitepoint.com/a-primer-on-using-flexbox-with-compass
http://www.sitepoint.com/compass-extensions-worth-a-try
http://www.sitepoint.com/an-overview-compass-1-0/
http://www.sitepoint.com/css-sprites-sass-compass
http://www.sitepoint.com/dont-use-compass-anymore
http://www.sitepoint.com/useful-compass-features-web-development/