Milligram provides a minimal setup of styles for a fast and clean starting point. Just it! Only 2kb gzipped! It's not about a UI framework. Specially designed for better performance and higher productivity with fewer properties to reset resulting in cleaner code. Hope you enjoy!
Neptune CSS is free and open source. It is a lightweight CSS framework and makes you webdesign faster. You can setup your project by changing a few variables.
Pure is ridiculously tiny. The entire set of modules clocks in at 3.5KB* minified and gzipped. Crafted with mobile devices in mind, it was important to us to keep our file sizes small, and every line of CSS was carefully considered. If you decide to only use a subset of these modules, you'll save even more bytes.