De Gutenberg à CSS "quand on parle d’édition multi-supports, on désigne la production de ces multiples manifestations (imprimée, numérique…) à partir d’une expression (le fameux single source), le plus souvent en mode automatisé."
You do not have to follow these rules every time. If you have a good reason to break any of them, do. But they are safe to follow every time.
CSS Lint points out problems with your CSS