How do frontend, backend and full-stack responsibilities differ?

Frontend — what users see and touch: component architecture, state management, rendering strategy (SSR, SSG, SPA), accessibility (WCAG), responsive and internationalised UI, and Core Web Vitals performance.

Backend — business logic, APIs, persistence, authentication/authorization, background jobs and integrations; it is the *trust boundary*, so all validation and permission checks live here.

Full-stack engineering spans both plus the data and deployment layers: designing end-to-end features, contracts between client and server (typed schemas), and the operational concerns that keep them healthy.

Professional standards: typed languages (TypeScript), schema validation at the boundary, accessible semantic HTML, keyboard support, error and loading states, and observability on both client and server. Never trust client-side checks for security.