Modern web browsers execute third-party code through asynchronous, event-driven mechanisms that provide no formal guarantees of execution reproducibility, state provenance, or verifiable state progression. As a result, client-side behavior is typically inferred from logs, heuristics, or post-hoc interpretation rather than derived from explicit, validated execution state transitions. This work introduces a deterministic state-transition architecture for client-side web execution. The proposed model formalizes execution state as an explicit, observable representation, derives deterministic deltas between successive states, and applies rule-bound validation prior to execution advancement. Only transitions that satisfy predefined deterministic rules are permitted to progress within the defined execution boundary. The architecture operates alongside existing browser environments without modifying browser engines or application logic. It observes execution, captures deterministic state snapshots, evaluates transitions through rule-bound validation, and records outcomes in an append-only audit record. The resulting execution traces are reproducible and independently verifiable, eliminating reliance on probabilistic inference or semantic interpretation for client-side execution analysis. The primary contribution of this work is the formalization of a deterministic execution model and corresponding validation architecture for browser environments. The approach is implementation-agnostic and demonstrated through a reference implementation. All claims are intentionally limited to client-side execution validation within a defined execution boundary and do not extend to server-side computation, network-layer behavior, or content semantics.