Web standards

UI Web Standards

Overview

Comprehensive guidance for building accessible, performant, and user-friendly web interfaces aligned with Vercel's Web Interface Guidelines. Covers accessibility compliance (WCAG 2.1 AA), component patterns, form handling, animations, typography, navigation, and anti-patterns to avoid.

When to Use

• Reviewing HTML, React, Vue, Svelte components for accessibility and standards compliance
• Building forms, buttons, links, modals, navigation, lists, and interactive elements
• Optimizing performance via virtualization, lazy loading, and animation handling
• Implementing animations, transitions, or motion-sensitive designs
• Handling dark mode, i18n, localization, or theme-aware components
• Debugging focus states, keyboard navigation, or screen reader announcements
• Designing UX copy, error messages, loading states, or empty states

When NOT to Use

• Styling decisions unrelated to accessibility, performance, or UX patterns
• Branding or visual design guidelines (use design system skill instead)
• Backend logic or server-side rendering patterns (use architecture reviews)
• Non-web platforms (native mobile, desktop apps, terminal UIs)

---

Accessibility

Semantic HTML First

Use semantic elements; never <div onClick> for interactive content:

• <button> for actions (submit, cancel, toggle, delete)
• <a> or <Link> for navigation (internal/external)
• <label> for form controls (inputs, checkboxes, radios, selects)
• <table> for tabular data (with <thead>, <tbody>, proper headers)
• <nav>, <main>, <header>, <footer>, <section>, <article> for structure

Anti-pattern:

<div onClick={handleClick} role="button">
  Click me
</div>

Correct:

<button onClick={handleClick}>
  Click me
</button>

ARIA Labels & Roles

• Icon-only buttons require aria-label="..." describing intent
• Form controls without visible labels require aria-label or aria-labelledby
• Inputs with labels: use <label htmlFor="id"> (clickable target) or aria-label
• Interactive elements: expose role via semantic HTML; use role="..." only when semantic element unavailable
• Decorative icons: apply aria-hidden="true" to skip in accessibility tree
• Dynamic content: async updates use aria-live="polite" (toasts, validation messages, status updates)
• Skip links: include skip-to-main link at top of page

Example:

{/* Icon-only button */}
<button aria-label="Close modal" onClick={closeModal}>
  <CloseIcon />
</button>

{/* Decorative icon */}
<span aria-hidden="true">✓</span>

{/* Form with label */}
<label htmlFor="email">Email</label>
<input id="email" type="email" name="email" />

{/* Live region for async updates */}
<div aria-live="polite" aria-atomic="true">
  {validationError}
</div>

Keyboard Navigation

• Interactive elements (<button>, <a>, <input>, etc.) are keyboard-focusable by default
• Custom interactive elements require onKeyDown or onKeyUp handlers
• Common patterns:
  • Button/Link: Space/Enter to activate
  • Checkbox: Space to toggle
  • Radio Group: Arrow keys to navigate, Space to select
  • Menu: Arrow keys + Escape to close
  • Modal: Escape to close (if permitted), Tab trapped to modal
  • Autocomplete: Arrow keys to navigate, Enter to select, Escape to close

Example:

const MyButton = ({ onClick, disabled }) => (
  <button
    onClick={onClick}
    onKeyDown={(e) => {
      if (e.key === "Enter" || e.key === " ") {
        e.preventDefault();
        onClick(e);
      }
    }}
    disabled={disabled}
  >
    Click or press Enter/Space
  </button>
);

Form Accessibility

• Every form control must have a visible label or aria-label
• Labels clickable via htmlFor attribute or label wrapping control
• Semantic input types: type="email", type="tel", type="url", type="number" (enables mobile keyboards, browser validation)
• Autocomplete: use meaningful autocomplete attribute (e.g., autocomplete="email", autocomplete="current-password")
• Spellcheck: disable for email, code, username: spellCheck={false}
• Error states:
  • Display error inline next to field
  • Focus first invalid field on submit
  • Use aria-describedby to link input to error: <input aria-describedby="email-error" /><div id="email-error">{error}</div>
• Checkboxes & Radios:
  • Label and control share single hit target (label wraps input or uses htmlFor)
  • Group related radios with <fieldset> and <legend>

Example:

<div>
  <label htmlFor="email">Email address</label>
  <input
    id="email"
    name="email"
    type="email"
    autoComplete="email"
    spellCheck={false}
    aria-describedby={error ? "email-error" : undefined}
    required
  />
  {error && <div id="email-error" style={{ color: "red" }}>{error}</div>}
</div>

{/* Checkbox with label as hit target */}
<label>
  <input type="checkbox" name="terms" required />
  I agree to the terms
</label>

{/* Radio group */}
<fieldset>
  <legend>Preferred contact method</legend>
  <label>
    <input type="radio" name="contact" value="email" />
    Email
  </label>
  <label>
    <input type="radio" name="contact" value="phone" />
    Phone
  </label>
</fieldset>

Images

• All <img> tags require alt text (descriptive) or alt="" (if purely decorative)
• Decorative images use alt="" and aria-hidden="true"
• Content images describe purpose/subject in alt text (e.g., "Team photo at 2024 conference", not just "photo")

Example:

{/* Content image */}
<img src="team.jpg" alt="Team photo at 2024 annual conference" />

{/* Decorative divider */}
<img src="divider.svg" alt="" aria-hidden="true" />

Headings & Structure

• Hierarchical headings from <h1> to <h6> (skip levels only deliberately)
• Single <h1> per page (page title)
• Include skip link before main content: <a href="#main-content">Skip to main content</a>
• Heading anchors: add scroll-margin-top to avoid overlap with fixed headers

Example:

<a href="#main-content" className="sr-only">Skip to main content</a>

<h1>Page Title</h1>
<section>
  <h2 id="features" style={{ scrollMarginTop: "80px" }}>Features</h2>
  {/* ... */}
</section>

<style>
  .sr-only {
    position: absolute;
    width: 1px;
    height: 1px;
    padding: 0;
    margin: -1px;
    overflow: hidden;
    clip: rect(0, 0, 0, 0);
    white-space: nowrap;
    border-width: 0;
  }
</style>

---

Focus States

Visible Focus Indicators

• Never remove outlines without visible replacement
• Prefer :focus-visible over :focus (avoids outline on mouse click, shows on keyboard)
• Required indicator: ring or border on focus; sufficient contrast (3:1 minimum)
• Style example: focus-visible:ring-2 focus-visible:ring-offset-2 focus-visible:ring-blue-500

Anti-pattern:

button {
  outline: none; /* ❌ Removes focus completely */
}

Correct:

button:focus-visible {
  outline: 2px solid #0066cc;
  outline-offset: 2px;
}

/* Tailwind equivalent */
button {
  @apply focus-visible:ring-2 focus-visible:ring-offset-2 focus-visible:ring-blue-500;
}

Compound Controls

• Groups of controls (tabs, segmented buttons, option groups) use :focus-within to show parent focus state
• Example: tab group highlights when any tab is focused

Example:

.tab-group:focus-within {
  border-color: #0066cc;
}

.tab-group button:focus-visible {
  outline: none; /* Outlined by parent :focus-within */
}

---

Forms

Validation & Error Handling

Submit button states:

• Enabled by default (accept input)
• Disabled + spinner while request in flight
• Re-enabled on success or error
• Clear label while loading (e.g., "Saving..." not spinner-only)

Error handling:

• Inline errors next to fields (not top of form)
• Focus first invalid field on submit
• Specific error messages with fix/next step (not "Invalid input")
• Clear syntax for required fields (asterisk or label text)

Example:

const [isLoading, setIsLoading] = useState(false);
const [errors, setErrors] = useState({});

const handleSubmit = async (e) => {
  e.preventDefault();
  setIsLoading(true);
  try {
    await submitForm(formData);
  } catch (err) {
    setErrors(err.validationErrors);
    // Focus first error field
    const firstErrorField = Object.keys(err.validationErrors)[0];
    document.getElementById(firstErrorField)?.focus();
  } finally {
    setIsLoading(false);
  }
};

return (
  <form onSubmit={handleSubmit}>
    <div>
      <label htmlFor="email">Email *</label>
      <input
        id="email"
        type="email"
        name="email"
        autoComplete="email"
        aria-describedby={errors.email ? "email-error" : undefined}
      />
      {errors.email && (
        <div id="email-error" role="alert">
          {errors.email} – Try a different email address.
        </div>
      )}
    </div>

    <button type="submit" disabled={isLoading}>
      {isLoading ? "Saving..." : "Save"}
    </button>
  </form>
);

Autocomplete & Input Handling

• Never block paste: avoid onPaste with preventDefault()
• Meaningful name attributes: assist password managers and form restoration
• Disable autocomplete for non-auth fields to prevent password manager popup: autoComplete="off"
• Enable for auth fields (email, password): let password manager assist
• inputmode attribute: hint at mobile keyboard (e.g., inputMode="email" for email-like fields)

Example:

{/* Auth field - allow password manager */}
<input
  type="email"
  name="email"
  autoComplete="email"
/>

{/* Non-auth field - disable password manager */}
<input
  type="text"
  name="search-query"
  autoComplete="off"
  inputMode="search"
/>

{/* Phone number */}
<input
  type="tel"
  name="phone"
  autoComplete="tel"
  inputMode="tel"
  placeholder="555-123-4567…"
/>

Placeholders

• Placeholders not labels: show example format, not required instruction
• End with … to indicate example: placeholder="[email protected]…"
• Short, descriptive: help user understand expected input

---

Animation

Respect Motion Preferences

Always honor prefers-reduced-motion:

/* Disable animations for users who prefer reduced motion */
@media (prefers-reduced-motion: reduce) {
  * {
    animation: none !important;
    transition: none !important;
  }
}

JavaScript check:

const prefersReducedMotion = window.matchMedia("(prefers-reduced-motion: reduce)").matches;

if (!prefersReducedMotion) {
  // Apply animation
}

Animation Best Practices

• Only animate transform and opacity (GPU-accelerated)
• Never transition: all – list properties explicitly
• Set transform-origin when rotating/scaling
• SVG animations: wrap with <g>, set transform-box: fill-box; transform-origin: center
• Interruptible: animations should respond to user input (e.g., close modal during fade-out)
• Reasonable duration: 200–400ms for micro-interactions, 500–800ms for page transitions
• Easing: use cubic-bezier or ease-in-out; avoid linear for motion

Anti-pattern:

/* ❌ Slow, janky, disrespects prefers-reduced-motion */
transition: all 2s linear;

Correct:

/* ✅ GPU-accelerated, respectful, smooth */
@media (prefers-reduced-motion: no-preference) {
  .fade-in {
    animation: fadeIn 0.3s ease-out;
  }
}

@keyframes fadeIn {
  from {
    opacity: 0;
    transform: translateY(-4px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

/* Explicit properties, not "all" */
transition: opacity 0.3s ease-out, transform 0.3s ease-out;

SVG example:

<svg viewBox="0 0 100 100" style={{ transformBox: "fill-box", transformOrigin: "center" }}>
  <g style={{ transform: "rotate(45deg)" }}>
    <circle cx="50" cy="50" r="40" />
  </g>
</svg>

---

Typography

Text Formatting

• Ellipsis: use … (HTML entity …), not ... (three periods)
• Quotes: use curly quotes " " (HTML entities “ ”), not straight "
• Non-breaking spaces for:
  • Measurements: 10 MB, 5 GB
  • Shortcuts: ⌘ K, Ctrl + K
  • Brand/product names: GitHub Copilot
  • Prevent orphaned words at line breaks

Example:

<p>Use ⌘&nbsp;K to open the command palette.</p>
<p>Download the file (50&nbsp;MB) for offline access.</p>
<p>{item.name}&nbsp;—&nbsp;{item.category}</p>

Number & Date Formatting

• Loading states end with …: "Loading…", "Saving…", not "Loading" or spinners-only
• Tabular numbers: use font-variant-numeric: tabular-nums for columns of numbers (ensures monospace alignment)
• Date/Time: use Intl.DateTimeFormat, never hardcoded formats
• Numbers: use Intl.NumberFormat with locale awareness

Example:

{/* Locale-aware date */}
const formattedDate = new Intl.DateTimeFormat("en-US", {
  year: "numeric",
  month: "long",
  day: "numeric",
}).format(new Date());

{/* Locale-aware number */}
const formattedNumber = new Intl.NumberFormat("en-US", {
  style: "currency",
  currency: "USD",
}).format(1234.56);

{/* Tabular numbers (aligned columns) */}
<table style={{ fontVariantNumeric: "tabular-nums" }}>
  <tr>
    <td>1,234.56</td>
  </tr>
</table>

---

Content Handling

Text Overflow

• Long text: apply truncate, line-clamp-* (Tailwind), or text-wrap: balance for headings
• Flex children: always set min-w-0 to allow child truncation (flex doesn't shrink below content size by default)
• Break long words: use break-words or word-break: break-word as fallback

Example:

{/* Truncate long email */}
<div style={{ overflow: "hidden", textOverflow: "ellipsis", whiteSpace: "nowrap" }}>
  [email protected]
</div>

{/* Line clamp */}
<p style={{ display: "-webkit-box", WebkitLineClamp: 2, WebkitBoxOrient: "vertical", overflow: "hidden" }}>
  Multi-line text truncated after 2 lines…
</p>

{/* Flex container with truncation */}
<div style={{ display: "flex", minWidth: 0 }}>
  <span style={{ overflow: "hidden", textOverflow: "ellipsis", whiteSpace: "nowrap" }}>
    Long text
  </span>
</div>

Empty States

• Gracefully handle empty lists, no results, no data scenarios
• Show message: "No items yet" with optional action (import, create, etc.)
• Avoid blank screens: empty state is content, not an error

Example:

{items.length === 0 ? (
  <div style={{ textAlign: "center", padding: "40px" }}>
    <p>No deployments yet.</p>
    <button onClick={openCreateDialog}>Create your first deployment</button>
  </div>
) : (
  {/* Items list */}
)}

Input Anticipation

• Anticipate short, average, and very long inputs
• Test with: real domain data, edge cases, longest valid inputs
• Adjust layout for mobile vs desktop (stack vs side-by-side)

---

Images

Image Dimensions & Optimization

• Every <img> must have explicit width and height (prevents Cumulative Layout Shift - CLS)
• Aspect ratio: specify as number (e.g., 16 / 9) or via CSS for <img> responsive scaling
• Lazy loading: below-fold images use loading="lazy"
• Above-fold: use priority (Next.js) or fetchpriority="high"
• Modern formats: serve WebP with fallback (via <picture> or srcset)

Example:

{/* Above-fold, priority */}
<img
  src="hero.jpg"
  alt="Hero image"
  width={1200}
  height={600}
  priority
/>

{/* Below-fold, lazy load */}
<img
  src="feature.jpg"
  alt="Feature overview"
  width={800}
  height={600}
  loading="lazy"
/>

{/* Responsive with aspect ratio */}
<img
  src="responsive.jpg"
  alt="Responsive image"
  width={400}
  height={300}
  style={{ aspectRatio: "4 / 3", width: "100%", height: "auto" }}
/>

{/* Modern formats */}
<picture>
  <source srcSet="image.webp" type="image/webp" />
  <source srcSet="image.jpg" type="image/jpeg" />
  <img src="image.jpg" alt="Fallback" width={400} height={300} />
</picture>

---

Performance

Virtualization

• Large lists (>50 items): virtualize with content-visibility: auto or library (e.g., virtua, react-window, react-virtual)
• Unvirtualized lists render off-screen DOM, causing layout thrashing and slow scrolling
• Virtual scroll: only render visible items + buffer

Example:

import { Virtualizer } from "virtua";

<Virtualizer>
  {items.map((item) => (
    <div key={item.id}>{item.name}</div>
  ))}
</Virtualizer>

{/* CSS-based virtualization */}
<div style={{ contentVisibility: "auto" }}>
  {/* Large list */}
</div>

Layout Thrashing

Anti-pattern: reading layout in render (causes forced reflows):

{/* ❌ Triggers layout recalculation every render */}
<div>
  {items.map((item) => {
    const height = document.getElementById(item.id)?.offsetHeight;
    return <div key={item.id} style={{ height }}>{item.name}</div>;
  })}
</div>

Correct: batch reads/writes or avoid measurements in render:

useLayoutEffect(() => {
  // Batch read
  const rect = containerRef.current?.getBoundingClientRect();
  // Batch write
  setLayout(rect);
}, []);

// Or use ResizeObserver for responsive measurements
useEffect(() => {
  const observer = new ResizeObserver(([entry]) => {
    setWidth(entry.contentRect.width);
  });
  observer.observe(containerRef.current);
  return () => observer.disconnect();
}, []);

Forms: Controlled vs Uncontrolled

• Prefer uncontrolled inputs (no state per keystroke)
• Controlled inputs must have cheap onChange handlers (avoid expensive computations per keystroke)
• Use defaultValue when form has initial state but input is uncontrolled

Example:

{/* Uncontrolled - simpler, more performant */}
<input type="text" defaultValue="initial" />

{/* Controlled - only if needed for real-time validation/masking */}
const [value, setValue] = useState("");
return (
  <input
    value={value}
    onChange={(e) => setValue(e.target.value)} {/* Keep cheap */}
  />
);

Font & Network Performance

• Critical fonts: preload with <link rel="preload" as="font" href="..." type="font/..." crossOrigin>
• Use font-display: swap to show fallback immediately (avoid invisible text while loading)
• CDN domains: warm with <link rel="preconnect" href="https://cdn.example.com">

Example:

<head>
  <link rel="preconnect" href="https://fonts.googleapis.com" />
  <link rel="preload" href="/font.woff2" as="font" type="font/woff2" crossOrigin />
  <style>
    @font-face {
      font-family: "CustomFont";
      src: url("/font.woff2") format("woff2");
      font-display: swap;
    }
  </style>
</head>

---

Navigation & State

URL as Source of Truth

• Query params reflect UI state: filters, tabs, pagination, expanded panels, sorting
• Sync state to URL (use nuqs, next/router, or similar library)
• Deep linking: user can share/bookmark URL and restore full state
• Back button works intuitively (doesn't require custom handler for simple navigation)

Example:

import { useQueryState } from "next-usp"; // or similar

export default function ProductList() {
  const [tab, setTab] = useQueryState("tab", { defaultValue: "all" });
  const [sort, setSort] = useQueryState("sort", { defaultValue: "name" });

  return (
    <div>
      <button onClick={() => setTab("featured")} data-active={tab === "featured"}>
        Featured
      </button>
      <select value={sort} onChange={(e) => setSort(e.target.value)}>
        <option value="name">Name</option>
        <option value="price">Price</option>
      </select>
      {/* URL: ?tab=featured&sort=price */}
    </div>
  );
}

Links & Navigation

• Use <a> or <Link> for navigation (enables Cmd/Ctrl+click, middle-click, new tab)
• Never onClick on <div> for navigation
• Button vs Link: button = action, link = navigation

Anti-pattern:

<div onClick={() => navigate("/page")}>Go to page</div>

Correct:

<a href="/page">Go to page</a>
{/* or Next.js */}
<Link href="/page">Go to page</Link>

Destructive Actions

• Require confirmation (modal/dialog) or undo window (toast with "Undo" button)
• Avoid accidental clicks: clear label ("Delete permanently", not "Delete"), secondary button style
• After destructive action: show success confirmation (not silent success)

Example:

const handleDelete = async () => {
  const confirmed = window.confirm("Are you sure? This cannot be undone.");
  if (!confirmed) return;

  try {
    await deleteItem(id);
    showToast("Item deleted", {
      action: "Undo",
      onAction: () => restoreItem(id),
    });
  } catch (err) {
    showToast("Failed to delete item", { type: "error" });
  }
};

return (
  <button onClick={handleDelete} style={{ background: "red", color: "white" }}>
    Delete permanently
  </button>
);

---

Touch & Interaction

Touch Targets

• Minimum 44×44px touch target (or larger on mobile)
• touch-action: manipulation: prevents double-tap zoom delay (safe on mobile)
• Intentional -webkit-tap-highlight-color: replace default gray highlight

Example:

button {
  min-width: 44px;
  min-height: 44px;
  touch-action: manipulation;
  -webkit-tap-highlight-color: rgba(0, 0, 0, 0.1);
}

Scrolling & Overflow

• Modal/drawer: overscroll-behavior: contain prevents scroll-through
• Full-bleed images: overflow-x: hidden on body, ensure content doesn't overflow
• Momentum scrolling (iOS): use -webkit-overflow-scrolling: touch (deprecated but still supported)

Example:

.modal {
  overscroll-behavior: contain;
  overflow-y: auto;
}

body {
  overflow-x: hidden;
}

Drag & Drop

• Disable text selection during drag: user-select: none
• Mark dragged elements inert to prevent nested interaction
• Visual feedback: cursor change, opacity, or highlight

Example:

const [isDragging, setIsDragging] = useState(false);

const handleDragStart = (e) => {
  setIsDragging(true);
  e.dataTransfer.effectAllowed = "move";
};

return (
  <div
    draggable
    onDragStart={handleDragStart}
    onDragEnd={() => setIsDragging(false)}
    style={{
      userSelect: "none",
      cursor: isDragging ? "grabbing" : "grab",
      opacity: isDragging ? 0.7 : 1,
    }}
  >
    Drag me
  </div>
);

Auto-focus

• Desktop only: auto-focus primary input on page load (good UX)
• Mobile: skip auto-focus (keyboard appears unexpectedly, obscures content)
• Single, clear purpose: avoid auto-focus in multi-input forms (confusing)

Example:

const isMobile = /iPhone|iPad|Android/.test(navigator.userAgent);

return (
  <input
    autoFocus={!isMobile}
    placeholder="Start typing…"
  />
);

---

Safe Areas & Layout

Notches & Safe Areas

• Full-bleed layouts (hero images, sidebars): account for notches using env(safe-area-inset-*)
• iPhone notch: left/top/right/bottom insets
• Padding: add padding for critical content

Example:

.header {
  padding-left: max(1rem, env(safe-area-inset-left));
  padding-right: max(1rem, env(safe-area-inset-right));
  padding-top: max(1rem, env(safe-area-inset-top));
}

body {
  padding-bottom: env(safe-area-inset-bottom);
}

Container Overflow

• overflow-x: hidden on containers to hide off-screen content
• Ensure content doesn't overflow horizontally (test responsive breakpoints)
• Flex/Grid: prefer layout algorithms over JS measurement

---

Dark Mode & Theming

Color Scheme & Defaults

• color-scheme: dark on <html> or root element (fixes scrollbar, input borders in dark mode)
• Explicit background-color and color on form inputs (Windows dark mode needs this)
• <meta name="theme-color"> matches page background color (affects browser UI)

Example:

<html style="color-scheme: dark">
  <head>
    <meta name="theme-color" content="#1a1a1a" />
  </head>
</html>

<style>
  input,
  select,
  textarea {
    background-color: #fff;
    color: #000;
  }

  @media (prefers-color-scheme: dark) {
    input,
    select,
    textarea {
      background-color: #222;
      color: #fff;
    }
  }
</style>

---

Localization & i18n

Date, Number, and Language Handling

• Intl.DateTimeFormat: never hardcode date formats
• Intl.NumberFormat: handle currency, thousands separators, percentages
• Accept-Language header or navigator.languages[0]**: detect locale (never IP-based)
• lang attribute on <html>: aids screen readers and spell-check

Example:

const userLocale = navigator.language; // "en-US", "fr-FR", etc.

const formattedDate = new Intl.DateTimeFormat(userLocale, {
  year: "numeric",
  month: "long",
  day: "numeric",
}).format(new Date());

const formattedCurrency = new Intl.NumberFormat(userLocale, {
  style: "currency",
  currency: "USD",
}).format(1234.56);

return (
  <html lang={userLocale.split("-")[0]}>
    <div>{formattedDate}</div>
    <div>{formattedCurrency}</div>
  </html>
);

---

Hydration Safety

Server-Side Rendering (SSR) & Hydration

• Inputs with value require onChange handler or use defaultValue for uncontrolled
• Avoid hydration mismatch for date/time (server renders different value than client)
• suppressHydrationWarning only as last resort for intentional client-only content

Anti-pattern (hydration mismatch):

{/* Server renders "Fri Mar 30 2026", client renders today's date */}
<div>{new Date().toLocaleDateString()}</div>

Correct:

const [isMounted, setIsMounted] = useState(false);

useEffect(() => setIsMounted(true), []);

return (
  <div>{isMounted ? new Date().toLocaleDateString() : null}</div>
);

---

Hover & Interactive States

Hover States

• Interactive elements (buttons, links) require :hover state (CSS or Tailwind)
• Hover increases contrast or changes color/shadow (clear feedback)
• Avoid relying on hover alone for important info (mobile has no hover)

Example:

button {
  background-color: #0066cc;
  transition: background-color 0.2s ease-out;
}

button:hover {
  background-color: #0052a3;
}

/* Tailwind */
button {
  @apply bg-blue-600 hover:bg-blue-700 transition-colors;
}

Active & Disabled States

• Active state (:active, .active): visual feedback for pressed button
• Disabled state: reduced opacity, cursor: not-allowed, no hover effect

Example:

button:disabled {
  opacity: 0.5;
  cursor: not-allowed;
}

button:disabled:hover {
  background-color: #0066cc; /* No change on hover when disabled */
}

---

Content & Copy

Writing Style

• Active voice: "Install the CLI" not "The CLI will be installed"
• Second person: "You can deploy" not "One can deploy" or "I can deploy"
• Avoid first person: "Submit your form" not "I will submit your form"
• Title Case for headings/buttons (Chicago Manual of Style)
• Numerals for quantities: "8 deployments" not "eight deployments"
• Specific button labels: "Save API Key" not "Continue"
• Ampersand (&) over "and" in space-constrained labels
• Errors include fix/next step: "Email invalid – try a different address" not "Invalid email"

Example:

{/* ✓ Active, second person, specific */}
<button>Save Your API Key</button>

{/* ✓ Error with fix */}
<div>Email invalid – Enter a different address.</div>

{/* ✓ Numerals, active */}
{count} deployments available

{/* ✓ Title Case */}
<h2>Build & Deploy</h2>

---

Anti-patterns to Flag

Anti-pattern	Issue	Fix
user-scalable=no or maximum-scale=1	Disables zoom; accessibility failure	Remove; allow pinch zoom
onPaste with preventDefault	Blocks paste; user frustration	Allow paste; validate input after
transition: all	Slow, unpredictable animations	List properties: transition: opacity 0.3s, transform 0.3s
outline-none without :focus-visible replacement	No focus indicator; keyboard navigation breaks	Add :focus-visible:ring-2 or equivalent
Inline onClick on <div>	Not a button semantically; fails accessibility	Use <button>
<div role="button"> with onClick	Fake button; missing keyboard handlers	Use <button>
Missing image width/height	Layout shift; poor LCP	Add explicit dimensions
Large array .map() without virtualization	Slow scroll; DOM bloat	Use virtua, react-window, or content-visibility: auto
Form control without label	Accessibility failure	Add <label> or aria-label
Icon button without aria-label	Unclear intent; screen reader says "button"	Add descriptive aria-label
Hardcoded date/time formats	Breaks in other locales	Use Intl.DateTimeFormat
Auto-focus without justification	Keyboard appears on mobile; confusing	Use autoFocus={!isMobile} or skip
color-scheme missing	Dark mode form inputs broken	Add color-scheme: dark to root
Decorative images without alt=""	Clutter accessibility tree	Add alt="" and aria-hidden="true"
Query params not in URL	Can't share state; no deep linking	Use nuqs or router to sync URL
display: none for accessibility	Content removed from layout and tree	Use visually hidden class (see Headings section)

---

Review Output Format

Group findings by file using file:line format (clickable in VS Code):

src/components/Button.tsx:12
❌ Icon-only button missing aria-label: <button><CloseIcon /></button>
Fix: Add aria-label="Close modal"

src/pages/form.tsx:34
❌ Inputs without labels or aria-label
Fix: Add <label htmlFor="email"> or aria-label="Email address"

src/styles/globals.css:2
❌ outline-none without :focus-visible replacement
Fix: Add :focus-visible { outline: 2px solid #0066cc; }

src/components/List.tsx:5
⚠ Large array .map() without virtualization (250+ items)
Suggest: Add content-visibility: auto or use virtua library

Terse descriptions. Skip explanation unless fix non-obvious. No preamble.

---

Review Checklist

Accessibility (WCAG 2.1 AA)

• Semantic HTML: <button>, <a>, <label> used correctly
• Icon-only buttons have aria-label
• Form inputs have visible labels or aria-label
• Keyboard navigation works (Tab, Enter, Space, Arrow keys, Escape)
• Focus indicators visible (:focus-visible or ring-*)
• Images have alt text (descriptive) or alt="" (decorative)
• Decorative icons have aria-hidden="true"
• Async updates use aria-live="polite"
• Color not sole means of conveying info (test grayscale)
• Contrast ratio ≥ 4.5:1 for normal text, ≥ 3:1 for large text
• Headings hierarchical (<h1>–<h6> in order)

Forms

• Inputs have name and autoComplete attributes
• Semantic input types (email, tel, number, url)
• Paste not blocked (onPaste event doesn't preventDefault)
• Error messages inline, focused on submit
• Submit button disabled during request
• Non-auth fields have autoComplete="off"

Performance

• Images have explicit width and height
• Below-fold images have loading="lazy"
• Lists >50 items virtualized
• Animations use transform/opacity only
• No transition: all
• Fonts use font-display: swap

Animation & Motion

• prefers-reduced-motion respected
• Animations interruptible
• Duration reasonable (200–800ms)

Dark Mode & Theming

• color-scheme: dark on root
• Form inputs have explicit colors in dark mode
• <meta name="theme-color"> set

Localization

• Dates use Intl.DateTimeFormat
• Numbers use Intl.NumberFormat
• lang attribute on <html>
• No hardcoded format strings

Typography & Content

• Ellipsis (…), not ...
• Curly quotes (" "), not straight "
• Loading states end with …
• Button labels specific and active voice
• Error messages include fix/next step

Navigation & State

• Query params reflect UI state
• Links use <a> or <Link> (not <div onClick>)
• Destructive actions require confirmation

Touch & Mobile

• Touch targets ≥ 44×44px
• touch-action: manipulation set
• Auto-focus skipped on mobile
• Safe areas considered (env(safe-area-inset-*))

Hydration (SSR)

• No hydration mismatch (date/time values)
• Controlled inputs have onChange
• Uncontrolled inputs use defaultValue