Audits and enforces web interface standards covering accessibility, performance, UX, and code quality from Vercel Engineering's Web Interface Guidelines. Use this skill when reviewing UI code, checking accessibility compliance, auditing design implementations, running pre-merge frontend checks, or when someone asks to "review my UI", "check accessibility", "check my site", "audit design", or "is this component correct". Apply proactively when writing form elements, interactive components, images, animations, or anything that users directly touch — these guidelines prevent common mistakes before they reach production.
Audits and enforces web interface standards covering accessibility, performance, UX, and code quality from Vercel Engineering's Web Interface Guidelines. Use this skill when reviewing UI code, checking accessibility compliance, auditing design implementations, running pre-merge frontend checks, or when someone asks to "review my UI", "check accessibility", "check my site", "audit design", or "is this component correct". Apply proactively when writing form elements, interactive components, images, animations, or anything that users directly touch — these guidelines prevent common mistakes before they reach production.
Web Design Guidelines
Good UI code is more than styling — it's accessibility, resilience, performance, and copy, all working together. These guidelines are derived from Vercel Engineering's Web Interface Guidelines, covering 18 categories that address the most common quality gaps in production web UIs.
Workflow
Read the relevant category file from references/ based on what's being reviewed (accessibility issues → references/accessibility.md, performance → references/performance-navigation.md, etc.)
Scan the code for violations in the relevant categories
Report findings in file:line format with the rule name, what's wrong, and the fix
Prioritize — fix Accessibility and Forms first (user-blocking), then Performance, then the rest
Read references/accessibility.md first for any UI review — it covers the most critical rules.
Category Overview
1. Accessibility — CRITICAL
Interactive elements need accessible labels. Screen readers, voice navigation, and keyboard users depend on them. Every <button>, <a>, <input>, and icon-only element needs an accessible name.
Critical rules: aria-label on icon buttons, semantic HTML (<button> not <div onClick>), onKeyDown alongside onClick for custom interactive elements, skip navigation links.
Read references/accessibility.md for the complete rule set.
2. Focus States — CRITICAL
Every interactive element needs a visible focus indicator. Removing outline without providing a replacement breaks keyboard navigation entirely.
Critical rules: Never outline: none without focus-visible replacement; use :focus-visible (not :focus) to show indicators only for keyboard navigation.
Read references/accessibility.md for focus state rules.
3. Forms — HIGH
Forms are where users do work. Broken forms (missing autocomplete, wrong input types, unclear errors) lose users.
Critical rules: autocomplete attributes on all relevant inputs, correct type for inputs (email, tel, number), inline error messages next to the field (not toast-only), warn before navigating away from unsaved form state.
Read references/accessibility.md for form accessibility rules.
4. Animation — HIGH
Animations must respect user preferences and only animate GPU-friendly properties. Animating height, width, or top/left causes layout thrash on every frame.
Critical rules: @media (prefers-reduced-motion: reduce) wrapping all animations, animate only transform and opacity (not layout properties).
Read references/motion-typography.md for animation rules.
5. Typography — MEDIUM-HIGH
Typographic details signal quality. Wrong quote characters, missing number formatting, and unbalanced headings are visible to users even if they can't articulate why something looks off.
Critical rules: curly quotes (" not "), proper ellipsis (…), font-variant-numeric: tabular-nums for numbers in tables, text-wrap: balance for headings.
Read references/motion-typography.md for typography rules.
6. Content Handling — MEDIUM-HIGH
Text overflows, empty states, and long content need explicit handling. Without them, layouts break in production with real data.
Critical rules: overflow: hidden; text-overflow: ellipsis on truncated text, min-width: 0 on flex children that truncate, design explicit empty states (not blank areas), handle long unbroken strings with overflow-wrap: break-word.
Read references/content-media.md for content handling rules.
7. Images — MEDIUM-HIGH
Images without dimensions cause layout shift; images without lazy loading degrade initial load performance.
Critical rules: explicit width and height attributes on all <img> tags, loading="lazy" on below-fold images, fetchpriority="high" on the above-fold hero image.
Read references/content-media.md for image rules.
8. Performance — MEDIUM
Perceived performance is about loading the right things at the right time. Virtualizing long lists and preloading critical resources are the highest-impact moves.
Critical rules: virtualize lists over 50 items (react-virtual, react-window), <link rel="preconnect"> for third-party origins, <link rel="preload"> for critical fonts.
Read references/performance-navigation.md for performance rules.
9. Navigation & State — MEDIUM
URL should reflect app state. If refreshing the page loses the user's context (search query, selected tab, filters), that's a bug.
Critical rules: URL reflects current state (query params for filters/search), stateful UI is deep-linkable, browser back button works as expected.
Read references/performance-navigation.md for navigation rules.
10. Touch & Interaction — MEDIUM
Mobile users tap; desktop users click. Both need appropriate interaction handling.
Critical rules: touch-action: manipulation on interactive elements (prevents 300ms tap delay), overscroll-behavior: contain on scrollable containers to prevent scroll chaining.
Read references/platform-ux.md for touch and interaction rules.
11. Safe Areas — MEDIUM
Full-bleed layouts on mobile need to respect device safe areas (notch, home indicator). Content clipped by the notch is inaccessible.
Critical rules: env(safe-area-inset-*) CSS variables for full-bleed layouts, viewport-fit=cover in the viewport meta tag when using safe area insets.
Read references/platform-ux.md for safe area rules.
12. Dark Mode — MEDIUM
Dark mode needs explicit handling — not just color inversion. System color scheme should be respected by default.
Critical rules: color-scheme: dark on <html> or in CSS for dark themes, <meta name="theme-color"> for browser chrome, no hardcoded hex colors in dark mode styles (use CSS variables).
Read references/platform-ux.md for dark mode rules.
13. Locale & i18n — MEDIUM
Date, number, and currency formatting must use the browser's locale APIs. Hardcoded formats break for international users.
Critical rules: Intl.DateTimeFormat for date formatting (not manual string concatenation), Intl.NumberFormat for numbers and currency, never hardcode locale-specific formats.
Read references/platform-ux.md for locale rules.
14. Hydration — MEDIUM
React hydration mismatches cause content flicker or errors. The most common cause is rendering client-only values (Date.now(), Math.random(), localStorage) on the server.
Critical rules: controlled inputs (<input value={...}>) need an onChange handler, avoid rendering client-specific values in server-rendered markup.
Read references/platform-ux.md for hydration rules.
15. Hover & Interactive States — LOW-MEDIUM
Every interactive element needs a hover state to signal clickability. Missing hover states make UI feel unresponsive.
Critical rules: all <button> and <a> elements have :hover styles, cursor changes (cursor: pointer) on interactive elements.
Read references/platform-ux.md for hover state rules.
16. Content & Copy — LOW-MEDIUM
Words are UI. Active voice, specific labels, and correct casing make interfaces feel professional. Passive and vague copy signals low quality.
Critical rules: active voice in button labels ("Save changes" not "Changes will be saved"), Title Case for headings, specific CTAs ("Download invoice" not "Click here").
Read references/platform-ux.md for copy guidelines.
17. Anti-patterns — LOW
Common patterns that appear in code reviews that should be rejected outright, regardless of context.
Read references/antipatterns.md for the complete list.
Output Format
When auditing code, report each issue as:
[CATEGORY] [PRIORITY] rule-name
File: path/to/component.tsx:42
Issue: <img> missing explicit width and height attributes
Fix: Add width={800} height={600} to prevent layout shift during image load
Group findings by category, ordered by priority (CRITICAL first).
Quality Checklist
All icon-only buttons have aria-label
No outline: none without :focus-visible replacement
All <input> elements have autocomplete attributes where relevant
All animations wrapped in @media (prefers-reduced-motion: reduce)
All <img> have explicit width and height
Lists over 50 items are virtualized
URL reflects filterable/searchable state
References
references/accessibility.md — Accessibility, Focus States, Forms (categories 1-3)
