Single Reaponsibility principal. Create a class called .button. The responsibility of that class is to define what a button looks like. Then create a class call

Single Reaponsibility principal. Create a class called .button. The responsibility of that class is to define what a button looks like. Then create a class called .button--primary whose sole responsibility is to define what a primary button looks like. If you create a class called .primary it has multiple responsibilities spread over the solution making it harder to maintain and improve upon