accessibility

# 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-l