accessibility

What this skill does

# Accessibility (a11y)

Comprehensive accessibility guidelines based on WCAG 2.1 and Lighthouse accessibility audits. Goal: make content usable by everyone, including people with disabilities.

## WCAG Principles: POUR

| Principle | Description |
|-----------|-------------|
| **P**erceivable | Content can be perceived through different senses |
| **O**perable | Interface can be operated by all users |
| **U**nderstandable | Content and interface are understandable |
| **R**obust | Content works with assistive technologies |

## Conformance levels

| Level | Requirement | Target |
|-------|-------------|--------|
| **A** | Minimum accessibility | Must pass |
| **AA** | Standard compliance | Should pass (legal requirement in many jurisdictions) |
| **AAA** | Enhanced accessibility | Nice to have |

---

## Perceivable

### Text alternatives (1.1)

**Images require alt text:**
```html
<!-- ❌ Missing alt -->
<img src="chart.png">

<!-- ✅ Descriptive alt -->
<img src="chart.png" alt="Bar chart showing 40% increase in Q3 sales">

<!-- ✅ Decorative image (empty alt) -->
<img src="decorative-border.png" alt="" role="presentation">

<!-- ✅ Complex image with longer description -->
<figure>
  <img src="infographic.png" alt="2024 market trends infographic" 
       aria-describedby="infographic-desc">
  <figcaption id="infographic-desc">
    <!-- Detailed description -->
  </figcaption>
</figure>
```

**Icon buttons need accessible names:**
```html
<!-- ❌ No accessible name -->
<button><svg><!-- menu icon --></svg></button>

<!-- ✅ Using aria-label -->
<button aria-label="Open menu">
  <svg aria-hidden="true"><!-- menu icon --></svg>
</button>

<!-- ✅ Using visually hidden text -->
<button>
  <svg aria-hidden="true"><!-- menu icon --></svg>
  <span class="visually-hidden">Open menu</span>
</button>
```

**Visually hidden class:**
```css
.visually-hidden {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border: 0;
}
```

### Color contrast (1.4.3, 1.4.6)

| Text Size | AA minimum | AAA enhanced |
|-----------|------------|--------------|
| Normal text (< 18px / < 14px bold) | 4.5:1 | 7:1 |
| Large text (≥ 18px / ≥ 14px bold) | 3:1 | 4.5:1 |
| UI components & graphics | 3:1 | 3:1 |

```css
/* ❌ Low contrast (2.5:1) */
.low-contrast {
  color: #999;
  background: #fff;
}

/* ✅ Sufficient contrast (7:1) */
.high-contrast {
  color: #333;
  background: #fff;
}

/* ✅ Focus states need contrast too */
:focus-visible {
  outline: 2px solid #005fcc;
  outline-offset: 2px;
}
```

**Don't rely on color alone:**
```html
<!-- ❌ Only color indicates error -->
<input class="error-border">
<style>.error-border { border-color: red; }</style>

<!-- ✅ Color + icon + text -->
<div class="field-error">
  <input aria-invalid="true" aria-describedby="email-error">
  <span id="email-error" class="error-message">
    <svg aria-hidden="true"><!-- error icon --></svg>
    Please enter a valid email address
  </span>
</div>
```

### Media alternatives (1.2)

```html
<!-- Video with captions -->
<video controls>
  <source src="video.mp4" type="video/mp4">
  <track kind="captions" src="captions.vtt" srclang="en" label="English" default>
  <track kind="descriptions" src="descriptions.vtt" srclang="en" label="Descriptions">
</video>

<!-- Audio with transcript -->
<audio controls>
  <source src="podcast.mp3" type="audio/mp3">
</audio>
<details>
  <summary>Transcript</summary>
  <p>Full transcript text...</p>
</details>
```

---

## Operable

### Keyboard accessible (2.1)

**All functionality must be keyboard accessible:**
```javascript
// ❌ Only handles click
element.addEventListener('click', handleAction);

// ✅ Handles both click and keyboard
element.addEventListener('click', handleAction);
element.addEventListener('keydown', (e) => {
  if (e.key === 'Enter' || e.key === ' ') {
    e.preventDefault();
    handleAction();
  }
});
```

**No keyboard traps:**
```javascript
// Modal focus management
function openModal(modal) {
  const focusableElements = modal.querySelectorAll(
    'button, [href], input, select, textarea, [tabindex]:not([tabindex="-1"])'
  );
  const firstElement = focusableElements[0];
  const lastElement = focusableElements[focusableElements.length - 1];
  
  // Trap focus within modal
  modal.addEventListener('keydown', (e) => {
    if (e.key === 'Tab') {
      if (e.shiftKey && document.activeElement === firstElement) {
        e.preventDefault();
        lastElement.focus();
      } else if (!e.shiftKey && document.activeElement === lastElement) {
        e.preventDefault();
        firstElement.focus();
      }
    }
    if (e.key === 'Escape') {
      closeModal();
    }
  });
  
  firstElement.focus();
}
```

### Focus visible (2.4.7)

```css
/* ❌ Never remove focus outlines */
*:focus { outline: none; }

/* ✅ Use :focus-visible for keyboard-only focus */
:focus {
  outline: none;
}

:focus-visible {
  outline: 2px solid #005fcc;
  outline-offset: 2px;
}

/* ✅ Or custom focus styles */
button:focus-visible {
  box-shadow: 0 0 0 3px rgba(0, 95, 204, 0.5);
}
```

### Skip links (2.4.1)

```html
<body>
  <a href="#main-content" class="skip-link">Skip to main content</a>
  <header><!-- navigation --></header>
  <main id="main-content" tabindex="-1">
    <!-- main content -->
  </main>
</body>
```

```css
.skip-link {
  position: absolute;
  top: -40px;
  left: 0;
  background: #000;
  color: #fff;
  padding: 8px 16px;
  z-index: 100;
}

.skip-link:focus {
  top: 0;
}
```

### Timing (2.2)

```javascript
// Allow users to extend time limits
function showSessionWarning() {
  const modal = createModal({
    title: 'Session Expiring',
    content: 'Your session will expire in 2 minutes.',
    actions: [
      { label: 'Extend session', action: extendSession },
      { label: 'Log out', action: logout }
    ],
    timeout: 120000 // 2 minutes to respond
  });
}
```

### Motion (2.3)

```css
/* Respect reduced motion preference */
@media (prefers-reduced-motion: reduce) {
  *,
  *::before,
  *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
    scroll-behavior: auto !important;
  }
}
```

---

## Understandable

### Page language (3.1.1)

```html
<!-- ❌ No language specified -->
<html>

<!-- ✅ Language specified -->
<html lang="en">

<!-- ✅ Language changes within page -->
<p>The French word for hello is <span lang="fr">bonjour</span>.</p>
```

### Consistent navigation (3.2.3)

```html
<!-- Navigation should be consistent across pages -->
<nav aria-label="Main">
  <ul>
    <li><a href="/" aria-current="page">Home</a></li>
    <li><a href="/products">Products</a></li>
    <li><a href="/about">About</a></li>
  </ul>
</nav>
```

### Form labels (3.3.2)

```html
<!-- ❌ No label association -->
<input type="email" placeholder="Email">

<!-- ✅ Explicit label -->
<label for="email">Email address</label>
<input type="email" id="email" name="email" 
       autocomplete="email" required>

<!-- ✅ Implicit label -->
<label>
  Email address
  <input type="email" name="email" autocomplete="email" required>
</label>

<!-- ✅ With instructions -->
<label for="password">Password</label>
<input type="password" id="password" 
       aria-describedby="password-requirements">
<p id="password-requirements">
  Must be at least 8 characters with one number.
</p>
```

### Error handling (3.3.1, 3.3.3)

```html
<!-- Announce errors to screen readers -->
<form novalidate>
  <div class="field" aria-live="polite">
    <label for="email">Email</label>
    <input type="email" id="email" 
           aria-invalid="true"
           aria-describedby="email-error">
    <p id="email-error" class="error" role="alert">
      Please enter a valid email address (e.g., [email protected])
    </p>
  </div>
</form>
```

```javascript
// Focus first error on submit
form.addEventListener('submit', (e) => {
  const firstError = f