magnus919_agent-skills/frontend-engineering/SKILL.md

name, description, license, metadata

name	description	license	metadata
frontend-engineering	Build and maintain web frontends — component architecture, state management, API integration, responsive layout, client-side performance, and frontend testing patterns. Framework agnostic, focused on web frontend implementation. Do not use for backend service implementation, data engineering, or platform infrastructure work.	MIT	tags source_repo frontend, web, ui, components, state-management, performance, javascript, typescript, responsive, testing https://github.com/magnus919/hermes-profiles

Frontend Engineering Methodology

Frontend engineering is the craft of building the user-facing layer of applications — components, state management, API integration, responsive layout, and client-side performance. This methodology bridges UX design (user journeys, wireframes, accessibility standards) and quality validation (qa-methodology).

The Frontend Engineer's Domain

You own	You don't own
Component implementation — UI component composition, props/state interfaces, rendering patterns, lifecycle	User journeys, wireframes, accessibility standards, interaction design — that's the product-design-and-ux
State management — client-side state architecture, data fetching patterns, caching, optimistic updates	API contract design — that's the api-design-and-evolution
API integration — frontend-to-backend data flow, auth flows (OAuth, JWT), real-time updates	Test strategy and automation — that's the QA-engineer
Responsive design implementation — layout systems, breakpoints, cross-device testing	Visual identity and brand guidelines — that's the brand-designer
Client-side performance — bundle optimization, lazy loading, Core Web Vitals, render optimization	Editorial content and copy — owned by the content and marketing team, outside this skill's scope
Frontend testing — component tests, integration tests, visual regression, accessibility tests	Code review and quality gates — that's the qa-methodology
E2E test scenarios and user-flow coverage for frontend features	Operating the browser test tool (Playwright) — authoring/running specs, selectors, network mocking, scraping — route to playwright
Mobile app implementation (iOS/Android/Flutter/React Native)	Mobile platform work — scaffolding, builds and code signing, device/emulator testing, store submission, mobile lifecycle — route to mobile-development; this skill owns web frontends
Build tooling — bundler config, TypeScript config, linting, formatting, dev environment	CI/CD pipeline infrastructure — that's the platform-engineer

For React component and hooks work, load react; for Vite config, modes, plugins, and builds, load vite. This skill remains the owner of framework-neutral architecture and frontend strategy.

Reference Files

Reference	When to load
references/component-architecture.md	Designing component trees — composition patterns, props/state interfaces, lifecycle, accessibility fundamentals
references/state-management.md	Choosing and implementing state management — client vs server state, data fetching, caching, optimistic updates
references/api-integration.md	Connecting frontend to backend — API client design, auth token flow, error handling in the UI, real-time subscriptions
references/responsive-layout-testing.md	Implementing responsive designs (layout system selection — Grid vs Flexbox vs Container Queries, breakpoint strategies, cross-device testing methodology) and testing frontend code (component testing with Testing Library, integration testing with Playwright/Cypress, visual regression, accessibility testing with axe-core and Lighthouse CI, test data management)
references/performance.md	Optimizing client-side performance — Core Web Vitals, bundle analysis, code splitting, render optimization

Templates

Template	When to Use
templates/component-state-design-record.md	Designing a component tree and state ownership for a feature — decomposition, state scoping, data fetching, and error/loading UX
templates/performance-budget.md	Defining performance targets — bundle byte budgets, Core Web Vitals budgets, measurement setup, and CI enforcement

Scripts

Script	When to Use
scripts/bundle-budget-checker.py	Checking a bundle size report against total and per-chunk budgets; fails (exit 1) when a budget is exceeded, so CI can block performance regressions

Core Principles

Components are the unit of composition, not pages — Design and build components as reusable, composable units. Pages are assembled from components, not built as monoliths. A well-designed component can be reused in contexts its creator never imagined.

Co-locate state with the components that need it — Not every piece of state belongs in a global store. Local state stays local. Server state is fetched and cached. Only truly shared application state belongs in a global context.

Design for every state, not just the happy path — Every data-dependent component has at least four states: loading, empty, error, and success. Designing for all four is not a nicety — it creates a resilient user experience.

Accessibility is not a feature, it's a requirement — Keyboard navigation, screen reader support, color contrast, and focus management are not enhancements. They are part of the implementation contract.

Performance is a UX concern — Every millisecond of load time, every layout shift, every janky interaction erodes user trust. Performance budgeting, bundle analysis, and render optimization are part of frontend engineering, not an afterthought.