Imagine you have a root (let's say your index webpage). Then you can build a tree of what is used by what. So if you have a CSS classname that is used by C (C being an HTML element, component, whatever) and C is used by B, but B is not used by A (the root) then it's shaken (removed) and "falls" because it's not connected to the root.

Basically remove everything not connected to the root.