Skeleton UI is a collection of user interface utility styles built on top of the great Skeleton grid which is a "Beautiful Boilerplate for Responsive, Mobile-Friendly Development".
This package aims to be slim and as bloat-free as possible, but adds on a few easily customizeable UI elements.
Of course, Skeleton UI is responsive by default. However, with the power of SASS you can easily customize your gutters and columns and omit any classes you don't intend on using. Skeleton UI also includes a handy set of percentage-based fractional column classes to help in those tricky situations.
Responsive Device DemoSass makes CSS fun again. Sass is an extension of CSS3, adding nested rules, variables, mixins, selector inheritance, and more. It’s translated to well-formatted, standard CSS using the command line tool or a web-framework plugin.