CoreCSS is a minimal front-end web framework, built entirely in CSS with love by Patrick Griffiths, there is no CSS pre-processing and the source is well commented.  This framework was developed as a lightweight alternative to the larger frameworks out there, with the intent for 'you' the user to be able to be free to rapidly develop full websites and mockups with the ability to swap in and out any JS frameworks and or libraries that you see fit as well as build off of the CSS as you see fit.