Cirrus is a gorgeous CSS framework to simplify the process of designing more complex layouts and components. It is designed to be both flexible for developers to customize classes at a granular level and simplistic enough for quick prototyping.
Ranked in these QuestionsQuestion Ranking
Pro CSS Only
No additional JS/jQuery required to use.
Much smaller than Bootstrap with just as much flexibility.
Pro Mobile Responsive
Extremely responsive and supports many smaller screens.
Pro Open Source
Open source and quite actively maintained on Github.
Pro Supports Flexbox and CSS Grid
Supports both Flexbox and CSS grid making it a great modern choice for designing web apps.