CSS is a declarative language which basically means that it’s easy to make changes but very difficult to make sure those changes don’t produce unexpected consequences elsewhere in the project.
This site collects the current techniques and tools available for CSS testing to provide both a simple starting point and a general reference for future development.
Most CSS testing tools so far have been developed from scratch and cover the same ground repeatedly. By gathering this information here, we hope to create a recognisable vocabulary so that it becomes easier to find similar tools and encourage developers to take the tools and techniques here and build on top of them instead of starting from scratch.
Have a look at some of these videos. They might explain things better.