The subject of scaling CSS came up a lot in a recent ShopTalk Show with Ben Frain. Ben has put a lot of thought into the subject, even writing a complete book on it, Enduring CSS, which is centered around a whole ECSS methodology.
He talked about how there are essentially two solutions for styling at scale:
Total isolation
Total abstraction
Total isolation is some version of writing styles scoped to some boundary that you’ve set up (like a component) in which those styles don’t leak in or out.
Total abstraction is some version of writing styles that are global, yet so generic and re-usable, that they have no unintended side effects.
Total isolation might come from