Agent Skills: React 19 + Next.js 16 + Tailwind CSS Best Practices

React 19 + Next.js 16 + Tailwind CSS Best Practices

Comprehensive strict-rules reference for web apps built on React 19, the Next.js 16 App Router, and Tailwind CSS 4. Contains 34 rules across 6 categories. Each rule is stated as an Always/Never directive with a quantified impact, an incorrect example, and a correct example.

Stack Contract

All guidance assumes:

• React 19 with Server Components by default; Client Components only when interactivity is required ('use client' at the top)
• Next.js 16 App Router with app/ directory, layout.tsx, page.tsx, loading.tsx, error.tsx, not-found.tsx, parallel routes, intercepting routes
• Server Actions for mutations ('use server') — never useEffect for data fetching
• Tailwind CSS 4 with the @theme directive, dark: variant, container queries, and the standard 4pt spacing scale
• lucide-react as the canonical icon system
• No CSS-in-JS (no styled-components, no emotion) — Tailwind utility classes only, with cn() from clsx + tailwind-merge for conditional classes
• shadcn/ui primitives (Radix-based) preferred for dialogs, popovers, dropdowns, tooltips, toasts

When to Apply

Reference these rules when:

• Building any user-facing route, layout, or component
• Reviewing PRs for design / UX / accessibility regressions
• Choosing between modality types (dialog vs popover vs full-page)
• Implementing forms with Server Actions and useFormState / useOptimistic
• Configuring loading and error boundaries
• Designing onboarding, permissions, or settings flows
• Ensuring dark mode, focus management, and keyboard navigation work end-to-end

Rule Categories by Priority

| Priority | Category | Impact | Prefix | |----------|----------|--------|--------| | 1 | Navigation | CRITICAL | nav- | | 2 | Interaction Design | CRITICAL | inter- | | 3 | Accessibility | CRITICAL | acc- | | 4 | User Feedback | HIGH | feed- | | 5 | UX Patterns | HIGH | ux- | | 6 | Visual Design | HIGH | vis- |

Quick Reference

1. Navigation (CRITICAL)

• nav-primary - Use top nav (3-7 sections) or sidebar; never hamburger-only on desktop
• nav-app-router - Use App Router layouts, parallel routes, and <Link> for all internal navigation
• nav-page-actions - Place primary actions in the page header; never bury them in scroll

2. Interaction Design (CRITICAL)

• inter-touch-targets - 44×44 px minimum touch target (WCAG 2.5.5)
• inter-pointer-patterns - Use standard hover/click/long-press patterns; never invent new ones
• inter-microinteractions - Always confirm interaction with visual feedback within 100ms
• inter-keyboard-navigation - Every interactive element must be reachable and operable by keyboard
• inter-drag-drop - Provide a keyboard-accessible alternative whenever drag is offered
• inter-revalidation - Use revalidatePath/revalidateTag after mutations; never rely on client refresh
• inter-row-actions - Use a single "row action" pattern per list (kebab menu OR hover actions OR swipe)
• inter-search - Debounce search input by 200-300ms and reflect query in the URL

3. Accessibility (CRITICAL)

• acc-labels - Every interactive element has an accessible name
• acc-text-scaling - All text scales to 200% browser zoom without horizontal scroll
• acc-color-contrast - WCAG AA: 4.5:1 body text, 3:1 large/UI
• acc-reduce-motion - Respect prefers-reduced-motion: reduce
• acc-color-independent - Never rely on color alone to convey meaning
• acc-focus-management - Always render a visible focus ring; trap focus inside modals
• acc-relative-units - Use rem for text and spacing; never fix text size in px
• acc-responsive-layout - Every layout works at 320 px width without horizontal scroll

4. User Feedback (HIGH)

• feed-loading-states - Always use loading.tsx or <Suspense> with a skeleton matching final layout
• feed-error-states - Every route segment has error.tsx with a Try-Again action
• feed-toasts - Use toasts only for confirmations of non-blocking actions
• feed-success-confirmation - Confirm every destructive or irreversible action with explicit visible feedback
• feed-empty-states - Empty states explain why and offer the next action

5. UX Patterns (HIGH)

• ux-onboarding - Onboarding never exceeds 3 screens; always skippable
• ux-permissions - Request browser permissions in-context, not on page load
• ux-modality - Choose dialog / popover / full-page by content weight; never stack modals
• ux-destructive-confirmation - Destructive actions require a typed confirmation OR an undo window
• ux-data-entry - Use Server Actions + progressive enhancement; never disable submit while typing
• ux-undo - Prefer undo over confirmation for everyday actions
• ux-settings - Settings are autosaved on change; never gated behind a Save button

6. Visual Design (HIGH)

• vis-dark-mode - Use CSS custom properties + dark: variant; never hardcode text-black/bg-white
• vis-icon-system - Use lucide-react with 1.5px stroke and size-4/size-5 standard sizes
• vis-spacing - Use the Tailwind 4 pt scale and container queries; never use ad-hoc px margins

How to Use

Read individual reference files for detailed explanations and code examples:

• Section definitions - Category structure and impact levels
• Rule template - Template for adding new rules

Reference Files

| File | Description | |------|-------------| | references/_sections.md | Category definitions and ordering | | assets/templates/_template.md | Template for new rules |