The process of a designer handing off everything an engineer needs to actually build a finished screen — not just the visuals, but specs, states, and edge cases — done well, it's what keeps the shipped product looking like the design file instead of drifting from it.

A shared library of components, rules, and documentation that designers and engineers both reference, so questions like "which color, which spacing, what does this button look like" have one answer everyone can look up instead of re-deciding each time.