What is the best CSS framework?

A CSS framework is a set concepts and practices that deals with common tasks when writing CSS. A CSS framework speeds up the mock-up and development processes, makes sure code displays consistently across all browsers, helps keep code tidy, provides solutions to common CSS issues and can even teach good practices.

The best CSS frameworks have great documentation and community resources available to learn from, provide support for all major browsers, are flexible enough to adapt to almost all situations you may encounter and include good support for accessibility features.

Other important criteria to consider include legacy browser support, pre-processor support and whether a framework is ready to go out of the box with preset styles for various elements (buttons, forms, tooltips, etc) or if it aims to be lightweight as possible enabling you to build your own unique style on top of it without requiring lots of CSS overrides.