Thirdparty Web Interface Guidelines

Web Interface Guidelines

Third-party skill (upstream-maintained). See ATTRIBUTION.md.
Upstream: vercel-labs/web-interface-guidelines · vercel.com/design/guidelines · Local name: thirdparty-web-interface-guidelines

Review target files for compliance. Read files, check against rules below. Output concise but comprehensive—sacrifice grammar for brevity. High signal-to-noise.

When the user provides specific files or patterns, review those. Otherwise ask which files to review.

Rules

Accessibility

• Icon-only buttons need aria-label
• Form controls need <label> or aria-label
• Interactive elements need keyboard handlers (onKeyDown/onKeyUp)
• <button> for actions, <a>/<Link> for navigation (not <div onClick>)
• Images need alt (or alt="" if decorative)
• Decorative icons need aria-hidden="true"
• Async updates (toasts, validation) need aria-live="polite"
• Use semantic HTML (<button>, <a>, <label>, <table>) before ARIA
• Headings hierarchical <h1>–<h6>; include skip link for main content
• scroll-margin-top on heading anchors

Focus States

• Interactive elements need visible focus: focus-visible:ring-* or equivalent
• Never outline-none / outline: none without focus replacement
• Use :focus-visible over :focus (avoid focus ring on click)
• Group focus with :focus-within for compound controls

Forms

• Inputs need autocomplete and meaningful name
• Use correct type (email, tel, url, number) and inputmode
• Never block paste (onPaste + preventDefault)
• Labels clickable (htmlFor or wrapping control)
• Disable spellcheck on emails, codes, usernames (spellCheck={false})
• Checkboxes/radios: label + control share single hit target (no dead zones)
• Submit button stays enabled until request starts; spinner during request
• Errors inline next to fields; focus first error on submit
• Placeholders end with … and show example pattern
• autocomplete="off" on non-auth fields to avoid password manager triggers
• Warn before navigation with unsaved changes (beforeunload or router guard)

Animation

• Honor prefers-reduced-motion (provide reduced variant or disable)
• Animate transform/opacity only (compositor-friendly)
• Never transition: all—list properties explicitly
• Set correct transform-origin
• SVG: transforms on <g> wrapper with transform-box: fill-box; transform-origin: center
• Animations interruptible—respond to user input mid-animation

Typography

• … not ...
• Curly quotes " " not straight "
• Non-breaking spaces: 10 MB, ⌘ K, brand names
• Loading states end with …: "Loading…", "Saving…"
• font-variant-numeric: tabular-nums for number columns/comparisons
• Use text-wrap: balance or text-pretty on headings (prevents widows)

Content Handling

• Text containers handle long content: truncate, line-clamp-*, or break-words
• Flex children need min-w-0 to allow text truncation
• Handle empty states—don't render broken UI for empty strings/arrays
• User-generated content: anticipate short, average, and very long inputs

Images

• <img> needs explicit width and height (prevents CLS)
• Below-fold images: loading="lazy"
• Above-fold critical images: priority or fetchpriority="high"

Performance

• Large lists (>50 items): virtualize (virtua, content-visibility: auto)
• No layout reads in render (getBoundingClientRect, offsetHeight, offsetWidth, scrollTop)
• Batch DOM reads/writes; avoid interleaving
• Prefer uncontrolled inputs; controlled inputs must be cheap per keystroke
• Add <link rel="preconnect"> for CDN/asset domains
• Critical fonts: <link rel="preload" as="font"> with font-display: swap

Navigation & State

• URL reflects state—filters, tabs, pagination, expanded panels in query params
• Links use <a>/<Link> (Cmd/Ctrl+click, middle-click support)
• Deep-link all stateful UI (if uses useState, consider URL sync via nuqs or similar)
• Destructive actions need confirmation modal or undo window—never immediate

Touch & Interaction

• touch-action: manipulation (prevents double-tap zoom delay)
• -webkit-tap-highlight-color set intentionally
• overscroll-behavior: contain in modals/drawers/sheets
• During drag: disable text selection, inert on dragged elements
• autoFocus sparingly—desktop only, single primary input; avoid on mobile

Safe Areas & Layout

• Full-bleed layouts need env(safe-area-inset-*) for notches
• Avoid unwanted scrollbars: overflow-x-hidden on containers, fix content overflow
• Flex/grid over JS measurement for layout

Dark Mode & Theming

• color-scheme: dark on <html> for dark themes (fixes scrollbar, inputs)
• <meta name="theme-color"> matches page background
• Native <select>: explicit background-color and color (Windows dark mode)

Locale & i18n

• Dates/times: use Intl.DateTimeFormat not hardcoded formats
• Numbers/currency: use Intl.NumberFormat not hardcoded formats
• Detect language via Accept-Language / navigator.languages, not IP
• Brand names, code tokens, identifiers: wrap with translate="no" to prevent garbled auto-translation

Hydration Safety

• Inputs with value need onChange (or use defaultValue for uncontrolled)
• Date/time rendering: guard against hydration mismatch (server vs client)
• suppressHydrationWarning only where truly needed

Hover & Interactive States

• Buttons/links need hover: state (visual feedback)
• Interactive states increase contrast: hover/active/focus more prominent than rest

Content & Copy

• Active voice: "Install the CLI" not "The CLI will be installed"
• Title Case for headings/buttons (Chicago style)
• Numerals for counts: "8 deployments" not "eight"
• Specific button labels: "Save API Key" not "Continue"
• Error messages include fix/next step, not just problem
• Second person; avoid first person
• & over "and" where space-constrained

Anti-patterns (flag these)

• user-scalable=no or maximum-scale=1 disabling zoom
• onPaste with preventDefault
• transition: all
• outline-none without focus-visible replacement
• Inline onClick navigation without <a>
• <div> or <span> with click handlers (should be <button>)
• Images without dimensions
• Large arrays .map() without virtualization
• Form inputs without labels
• Icon buttons without aria-label
• Hardcoded date/number formats (use Intl.*)
• autoFocus without clear justification

Output Format

Group by file. Use file:line format (VS Code clickable). Terse findings.

## src/Button.tsx

src/Button.tsx:42 - icon button missing aria-label
src/Button.tsx:18 - input lacks label
src/Button.tsx:55 - animation missing prefers-reduced-motion
src/Button.tsx:67 - transition: all → list properties

## src/Modal.tsx

src/Modal.tsx:12 - missing overscroll-behavior: contain
src/Modal.tsx:34 - "..." → "…"

## src/Card.tsx

✓ pass

State issue + location. Skip explanation unless fix non-obvious. No preamble.

Source: vercel-labs/web-interface-guidelines · Vercel Web Interface Guidelines. Installed in cursor_kenji as thirdparty-web-interface-guidelines. See ATTRIBUTION.md.