accessibility（无障碍）

Web Accessibility (A11Y)

WCAG 2.1 AA compliance guidelines and implementation patterns.

WCAG Principles (POUR)

1. Perceivable - Information must be presentable in ways users can perceive
2. Operable - Interface must be operable by all users
3. Understandable - Information and operation must be understandable
4. Robust - Content must be robust enough for assistive technologies

Semantic HTML

Use Correct Elements

<!-- BAD: Div soup -->
<div class="header">
    <div class="nav">
        <div class="link" onclick="navigate()">Home</div>
    </div>
</div>

<!-- GOOD: Semantic elements -->
<header>
    <nav aria-label="Main navigation">
        <a href="/">Home</a>
    </nav>
</header>

Headings Hierarchy

<!-- BAD: Skipping heading levels -->
<h1>Page Title</h1>
<h3>Section</h3>  <!-- Skipped h2! -->

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

Landmark Regions

<header role="banner">
    <nav role="navigation" aria-label="Main">...</nav>
</header>

<main role="main">
    <article>...</article>
    <aside role="complementary">...</aside>
</main>

<footer role="contentinfo">...</footer>

Images & Media

Alternative Text

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

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

<!-- Complex image -->
<figure>
    <img src="diagram.png" alt="System architecture diagram">
    <figcaption>
        Figure 1: The system consists of a web server,
        application server, and database server.
    </figcaption>
</figure>

<!-- Logo/link -->
<a href="/">
    <img src="logo.png" alt="Company Name - Home">
</a>

Video & Audio

<video controls>
    <source src="video.mp4" type="video/mp4">
    <track kind="captions" src="captions.vtt" srclang="en" label="English">
    <track kind="descriptions" src="descriptions.vtt" srclang="en" label="Audio descriptions">
</video>

Forms

Labels

<!-- BAD: No label association -->
<input type="email" placeholder="Email">

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

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

Error Messages

<div class="form-group">
    <label for="email">Email address</label>
    <input
        type="email"
        id="email"
        name="email"
        aria-describedby="email-error email-hint"
        aria-invalid="true"
        required
    >
    <p id="email-hint" class="hint">We'll never share your email</p>
    <p id="email-error" class="error" role="alert">
        Please enter a valid email address
    </p>
</div>

Fieldsets & Legends

<fieldset>
    <legend>Shipping Address</legend>
    <label for="street">Street</label>
    <input type="text" id="street" name="street">
    <!-- More fields -->
</fieldset>

<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>

Keyboard Navigation

Focus Management

/* Never remove focus outline without replacement */
/* BAD */
*:focus { outline: none; }

/* GOOD: Custom focus styles */
:focus {
    outline: 2px solid #0066cc;
    outline-offset: 2px;
}

/* Focus-visible for keyboard-only focus */
:focus:not(:focus-visible) {
    outline: none;
}

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

Tab Order

<!-- Use natural tab order, avoid positive tabindex -->
<button>First</button>
<button>Second</button>
<button>Third</button>

<!-- Remove from tab order if visually hidden -->
<button tabindex="-1" aria-hidden="true">Skip</button>

Skip Links

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

<style>
.skip-link {
    position: absolute;
    left: -9999px;
}
.skip-link:focus {
    left: 0;
    top: 0;
    padding: 1rem;
    background: #000;
    color: #fff;
    z-index: 9999;
}
</style>

ARIA

ARIA Roles

<!-- Buttons that aren't <button> -->
<div role="button" tabindex="0" aria-pressed="false">Toggle</div>

<!-- Tab interface -->
<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>

<!-- Alert for dynamic content -->
<div role="alert" aria-live="assertive">Form submitted successfully</div>

ARIA States & Properties

<!-- Expanded/collapsed -->
<button aria-expanded="false" aria-controls="menu">Menu</button>
<ul id="menu" hidden>...</ul>

<!-- Current page -->
<nav>
    <a href="/" aria-current="page">Home</a>
    <a href="/about">About</a>
</nav>

<!-- Disabled -->
<button aria-disabled="true">Submit</button>

<!-- Loading -->
<button aria-busy="true">
    <span class="spinner" aria-hidden="true"></span>
    Loading...
</button>

Live Regions

<!-- Polite announcements (wait for pause) -->
<div aria-live="polite" aria-atomic="true">
    3 items in cart
</div>

<!-- Assertive announcements (immediate) -->
<div role="alert" aria-live="assertive">
    Error: Payment failed
</div>

<!-- Status updates -->
<div role="status" aria-live="polite">
    Showing results 1-10 of 100
</div>

Color & Contrast

Contrast Requirements

• Normal text: 4.5:1 minimum
• Large text (18pt+ or 14pt+ bold): 3:1 minimum
• UI components: 3:1 minimum

/* BAD: Low contrast */
.light-text {
    color: #999; /* On white: 2.85:1 - FAIL */
}

/* GOOD: Sufficient contrast */
.text {
    color: #595959; /* On white: 7:1 - PASS */
}

Don't Rely on Color Alone

<!-- BAD: Color only -->
<span class="error" style="color: red;">Invalid</span>

<!-- GOOD: Color + icon + text -->
<span class="error">
    <svg aria-hidden="true"><!-- Error icon --></svg>
    Error: Invalid email address
</span>

Testing

Automated Tools

# axe-core
npx @axe-core/cli https://example.com

# Lighthouse
npx lighthouse https://example.com --only-categories=accessibility

# Pa11y
npx pa11y https://example.com

Manual Testing Checklist

1. Keyboard-only navigation

- Tab through all interactive elements - Ensure visible focus indicator - Check logical tab order - Test all functionality without mouse

1. Screen reader testing

- VoiceOver (macOS): Cmd + F5 - NVDA (Windows): Free download - Test headings, links, forms, dynamic content

1. Zoom testing

- Zoom to 200%, check layout - Zoom to 400%, check readability

1. Color/contrast

- Use browser dev tools contrast checker - Test with color blindness simulators

React Accessibility

// Focus management
const inputRef = useRef(null);

useEffect(() => {
    if (showError) {
        inputRef.current?.focus();
    }
}, [showError]);

// Accessible component
function Modal({ isOpen, onClose, title, children }) {
    const modalRef = useRef(null);

    useEffect(() => {
        if (isOpen) {
            modalRef.current?.focus();
            document.body.style.overflow = 'hidden';
        }
        return () => {
            document.body.style.overflow = '';
        };
    }, [isOpen]);

    if (!isOpen) return null;

    return (
        <div
            role="dialog"
            aria-modal="true"
            aria-labelledby="modal-title"
            ref={modalRef}
            tabIndex={-1}
        >
            <h2 id="modal-title">{title}</h2>
            {children}
            <button onClick={onClose}>Close</button>
        </div>
    );
}

Common Issues & Fixes