frontend-engineering— agent skill

Overview

Frontend code touches every user. Poor UI engineering causes accessibility barriers, performance degradation, and broken experiences on non-standard devices. This skill enforces the discipline of building UI that works for everyone.

When to Use

• Building any user-facing UI component
• Reviewing frontend PRs
• Before any UI ships to production

Process

Step 1: Accessibility First

1. Every interactive element has a visible focus state.
2. All images have meaningful alt text (or alt="" for decorative).
3. Color contrast ratio ≥ 4.5:1 for normal text.
4. All functionality operable by keyboard alone.
5. Use semantic HTML: <button> not <div onclick>, <nav> not <div class="nav">.

Verify: Run axe DevTools or Lighthouse accessibility audit. Score ≥ 90.

Step 2: Performance (Core Web Vitals)

1. LCP (Largest Contentful Paint) < 2.5s: Optimize images, preload critical resources.
2. FID/INP < 100ms: Avoid long tasks on the main thread.
3. CLS (Cumulative Layout Shift) < 0.1: Reserve space for dynamic content.
4. Lazy load below-the-fold images and non-critical JS.
5. Bundle size: measure before and after. No unneeded dependencies.

Verify: Lighthouse performance score ≥ 80. CWV within targets.

Step 3: Responsive Design

1. Test at 320px, 768px, 1024px, 1440px viewports.
2. No horizontal scrolling at any standard viewport.
3. Touch targets ≥ 44px × 44px.

Step 4: State Management Discipline

1. Server state vs. client state are separate concerns — don't mix.
2. No prop drilling more than 2 levels — use context or state management.
3. Loading, error, and empty states handled for every async operation.

Verify: Loading, error, and empty states all visible in Storybook/dev environment.

Common Rationalizations (and Rebuttals)

Verification

• [ ] Accessibility audit score ≥ 90
• [ ] Core Web Vitals within targets
• [ ] Responsive at 320px/768px/1024px/1440px
• [ ] Loading, error, empty states handled
• [ ] All interactive elements keyboard-operable

References

• references/accessibility-checklist.md
• performance-optimization skill