accessibility（无障碍）

Accessibility (a11y)

Guidance for creating accessible web applications following WCAG guidelines.

Core Principles (POUR)

1. Perceivable: Content can be perceived by all senses
2. Operable: Interface can be operated by everyone
3. Understandable: Content and UI are understandable
4. Robust: Works with assistive technologies

Quick Wins

Images

<!-- Informative image -->
<img src="chart.png" alt="Sales grew 25% in Q4 2024">

<!-- Decorative image -->
<img src="decoration.png" alt="" role="presentation">

Buttons & Links

<!-- Good: Descriptive -->
<button>Add to cart</button>
<a href="/docs">View documentation</a>

<!-- Bad: Vague -->
<button>Click here</button>
<a href="/docs">Read more</a>

Forms

<label for="email">Email address</label>
<input type="email" id="email" aria-describedby="email-hint">
<span id="email-hint">We'll never share your email</span>

Headings

<!-- Proper hierarchy -->
<h1>Page Title</h1>
  <h2>Section</h2>
    <h3>Subsection</h3>
  <h2>Another Section</h2>

Color & Contrast

Minimum Contrast Ratios

Don't Rely on Color Alone

<!-- Bad: Color only -->
<span style="color: red">Error</span>

<!-- Good: Color + icon + text -->
<span style="color: red">
  <svg aria-hidden="true">...</svg>
  Error: Email is required
</span>

Keyboard Navigation

Focus Management

/* Visible focus indicator */
:focus {
  outline: 2px solid #2563EB;
  outline-offset: 2px;
}

/* Don't remove focus styles! */
:focus { outline: none; } /* BAD */

Tab Order

<!-- Natural order follows DOM -->
<nav>
  <a href="/">Home</a>      <!-- Tab 1 -->
  <a href="/about">About</a> <!-- Tab 2 -->
</nav>

<!-- Skip link for keyboard users -->
<a href="#main" class="skip-link">Skip to content</a>

Keyboard Shortcuts

• Tab: Move forward
• Shift+Tab: Move backward
• Enter/Space: Activate
• Escape: Close/cancel
• Arrow keys: Navigate within components

ARIA Basics

When to Use ARIA

1. First: Use semantic HTML
2. Then: Add ARIA only if needed

<!-- Semantic HTML (preferred) -->
<button>Submit</button>
<nav>...</nav>

<!-- ARIA when needed -->
<div role="button" tabindex="0">Submit</div>
<div role="navigation">...</div>

Common ARIA Attributes

<!-- Labeling -->
<button aria-label="Close dialog">×</button>
<div aria-labelledby="section-title">...</div>

<!-- State -->
<button aria-expanded="false">Menu</button>
<div aria-hidden="true">Decorative content</div>

<!-- Live regions -->
<div aria-live="polite">Status updates here</div>
<div role="alert">Error message</div>

Component Patterns

Modal Dialog

<div role="dialog" aria-modal="true" aria-labelledby="dialog-title">
  <h2 id="dialog-title">Confirm Action</h2>
  <p>Are you sure?</p>
  <button>Cancel</button>
  <button>Confirm</button>
</div>

Tabs

<div role="tablist">
  <button role="tab" aria-selected="true" aria-controls="panel1">Tab 1</button>
  <button role="tab" aria-selected="false" aria-controls="panel2">Tab 2</button>
</div>
<div role="tabpanel" id="panel1">Content 1</div>
<div role="tabpanel" id="panel2" hidden>Content 2</div>

Testing Checklist

Automated Testing

• Run axe or Lighthouse accessibility audit
• Check color contrast with WebAIM checker
• Validate HTML

Manual Testing

• Navigate with keyboard only (no mouse)
• Test with screen reader (VoiceOver, NVDA)
• Zoom to 200% - is content usable?
• Disable CSS - is content logical?

Screen Reader Testing

macOS: VoiceOver (Cmd + F5)
Windows: NVDA (free) or JAWS
Mobile: TalkBack (Android), VoiceOver (iOS)

Quick Reference