Additionally, I believe in a strong Style Guide approach from the start. A Style Guide will define common elements (inputs, modals, page structure, fonts, typeography, margins, spacing) so that engineers don't have to worry about pixel perfect design. Ideally there should be very few "design" scenarios, all defined in components. I've seen such effective design systems, that almost no CSS was needed. Not to mention it's a very fluid user experience, and complete redesigns are trivial. You don't have to hunt down the margin prop on every component.