Frontend system design is the system-design discipline applied to client-side architecture. The questions look different from backend system design — instead of "design Twitter's tweet storage", interviewers ask "design a real-time collaborative document editor", "design Instagram's feed", "design a search-as-you-type interface for an autocomplete API at Google scale".
The trade-offs are different too: state management between client and server, optimistic updates, conflict resolution (CRDTs vs OT), virtualization for large lists, image lazy-loading strategies, code-splitting boundaries, and how to handle offline-first vs eventual-consistency at the UI layer. Frontend system design interviews became standard at staff+ levels at FAANG starting around 2022.