Single Responsibility Principle

Drew Barontini:

Hopefully that gives you a glimpse into how you can apply the Single Responsibility Principle to your CSS modules. Limit the scope of your modules, keep the styles small, and make sure your modules are properly encapsulated to ensure that your styles are both maintainable and predictable. Use submodules, modifiers, and context classes to make sure that your modules are nice and flex-y.

A super easy to follow article which introduces the fundamentals of writing better CSS.