HTML5系列（11）-- Web 无障碍开发指南

前端技术探索系列：HTML5 Web 无障碍开发指南 ♿

致读者：构建人人可用的网络 👋

前端开发者们，

今天我们将深入探讨 Web 无障碍开发，学习如何创建一个真正包容、人人可用的网站。让我们一起为更多用户提供更好的网络体验。

ARIA 角色与属性 🎯

基础 ARIA 实现

<!-- 导航菜单示例 -->
<nav role="navigation" aria-label="主导航"><ul role="menubar"><li role="none"><a role="menuitem" href="/" aria-current="page">首页</a></li><li role="none"><button role="menuitem"aria-haspopup="true"aria-expanded="false"aria-controls="submenu-1">产品</button><ul id="submenu-1" role="menu" aria-hidden="true"><li role="none"><a role="menuitem" href="/products/new">最新产品</a></li></ul></li></ul>
</nav>

动态内容管理

class AccessibleComponent {constructor(element) {this.element = element;this.setupKeyboardNavigation();}// 设置键盘导航setupKeyboardNavigation() {this.element.addEventListener('keydown', (e) => {switch(e.key) {case 'Enter':case ' ':this.activate(e);break;case 'ArrowDown':this.navigateNext(e);break;case 'ArrowUp':this.navigatePrevious(e);break;case 'Escape':this.close(e);break;}});}// 更新 ARIA 状态updateARIAState(element, state, value) {element.setAttribute(`aria-${state}`, value);// 通知屏幕阅读器this.announceChange(`${state} 已更改为 ${value}`);}// 向屏幕阅读器通知变化announceChange(message) {const announcement = document.createElement('div');announcement.setAttribute('aria-live', 'polite');announcement.setAttribute('class', 'sr-only');announcement.textContent = message;document.body.appendChild(announcement);setTimeout(() => announcement.remove(), 1000);}
}

语义化增强 📝

表单无障碍实现

<form role="form" aria-label="注册表单"><div class="form-group"><label for="username" id="username-label">用户名<span class="required" aria-hidden="true">*</span></label><input type="text"id="username"name="username"requiredaria-required="true"aria-labelledby="username-label"aria-describedby="username-help"aria-invalid="false"><div id="username-help" class="help-text">请输入3-20个字符的用户名</div></div><div class="form-group"><label for="password">密码</label><div class="password-input"><input type="password"id="password"name="password"aria-label="密码"aria-describedby="password-requirements"><button type="button"aria-label="显示密码"aria-pressed="false"onclick="togglePassword()">👁️</button></div><div id="password-requirements" class="help-text">密码必须包含字母和数字，长度至少8位</div></div>
</form>

表单验证与反馈

class AccessibleForm {constructor(formElement) {this.form = formElement;this.setupValidation();}setupValidation() {this.form.addEventListener('submit', this.handleSubmit.bind(this));this.form.addEventListener('input', this.handleInput.bind(this));}handleInput(e) {const field = e.target;const isValid = field.checkValidity();field.setAttribute('aria-invalid', !isValid);if (!isValid) {this.showError(field);} else {this.clearError(field);}}showError(field) {const errorId = `${field.id}-error`;let errorElement = document.getElementById(errorId);if (!errorElement) {errorElement = document.createElement('div');errorElement.id = errorId;errorElement.className = 'error-message';errorElement.setAttribute('role', 'alert');field.parentNode.appendChild(errorElement);}errorElement.textContent = field.validationMessage;field.setAttribute('aria-describedby', `${field.getAttribute('aria-describedby') || ''} ${errorId}`.trim());}clearError(field) {const errorId = `${field.id}-error`;const errorElement = document.getElementById(errorId);if (errorElement) {errorElement.remove();const describedBy = field.getAttribute('aria-describedby').replace(errorId, '').trim();if (describedBy) {field.setAttribute('aria-describedby', describedBy);} else {field.removeAttribute('aria-describedby');}}}
}

辅助技术支持 🔍

颜色对比度检查

class ColorContrastChecker {constructor() {this.minimumRatio = 4.5; // WCAG AA 标准}// 计算相对亮度calculateLuminance(r, g, b) {const [rs, gs, bs] = [r, g, b].map(c => {c = c / 255;return c <= 0.03928? c / 12.92: Math.pow((c + 0.055) / 1.055, 2.4);});return 0.2126 * rs + 0.7152 * gs + 0.0722 * bs;}// 计算对比度calculateContrastRatio(color1, color2) {const l1 = this.calculateLuminance(...this.parseColor(color1));const l2 = this.calculateLuminance(...this.parseColor(color2));const lighter = Math.max(l1, l2);const darker = Math.min(l1, l2);return (lighter + 0.05) / (darker + 0.05);}// 解析颜色值parseColor(color) {const hex = color.replace('#', '');return [parseInt(hex.substr(0, 2), 16),parseInt(hex.substr(2, 2), 16),parseInt(hex.substr(4, 2), 16)];}// 检查对比度是否符合标准isContrastValid(color1, color2) {const ratio = this.calculateContrastRatio(color1, color2);return {ratio,passes: ratio >= this.minimumRatio,level: ratio >= 7 ? 'AAA' : ratio >= 4.5 ? 'AA' : 'Fail'};}
}

字体可读性增强

/* 基础可读性样式 */
:root {--min-font-size: 16px;--line-height-ratio: 1.5;--paragraph-spacing: 1.5rem;
}body {font-family: system-ui, -apple-system, sans-serif;font-size: var(--min-font-size);line-height: var(--line-height-ratio);text-rendering: optimizeLegibility;
}/* 响应式字体大小 */
@media screen and (min-width: 320px) {body {font-size: calc(var(--min-font-size) + 0.5vw);}
}/* 提高可读性的文本间距 */
p {margin-bottom: var(--paragraph-spacing);max-width: 70ch; /* 最佳阅读宽度 */
}/* 链接可访问性 */
a {text-decoration: underline;text-underline-offset: 0.2em;color: #0066cc;
}a:hover, a:focus {text-decoration-thickness: 0.125em;outline: 2px solid currentColor;outline-offset: 2px;
}/* 焦点样式 */
:focus {outline: 3px solid #4A90E2;outline-offset: 2px;
}/* 隐藏元素但保持可访问性 */
.sr-only {position: absolute;width: 1px;height: 1px;padding: 0;margin: -1px;overflow: hidden;clip: rect(0, 0, 0, 0);border: 0;
}

实践项目：无障碍审计工具 🛠️

审计工具实现

class AccessibilityAuditor {constructor() {this.issues = [];}// 运行完整审计async audit() {this.issues = [];// 检查图片替代文本this.checkImages();// 检查表单标签this.checkForms();// 检查标题层级this.checkHeadings();// 检查颜色对比度await this.checkColorContrast();// 检查键盘可访问性this.checkKeyboardAccess();return this.generateReport();}// 检查图片替代文本checkImages() {const images = document.querySelectorAll('img');images.forEach(img => {if (!img.hasAttribute('alt')) {this.addIssue('error', 'missing-alt', '图片缺少替代文本', img);}});}// 检查表单标签checkForms() {const inputs = document.querySelectorAll('input, select, textarea');inputs.forEach(input => {if (!input.id || !document.querySelector(`label[for="${input.id}"]`)) {this.addIssue('error', 'missing-label','表单控件缺少关联标签', input);}});}// 检查标题层级checkHeadings() {const headings = document.querySelectorAll('h1, h2, h3, h4, h5, h6');let lastLevel = 0;headings.forEach(heading => {const currentLevel = parseInt(heading.tagName[1]);if (currentLevel - lastLevel > 1) {this.addIssue('warning', 'heading-skip','标题层级跳过', heading);}lastLevel = currentLevel;});}// 检查颜色对比度async checkColorContrast() {const contrastChecker = new ColorContrastChecker();const elements = document.querySelectorAll('*');elements.forEach(element => {const style = window.getComputedStyle(element);const backgroundColor = style.backgroundColor;const color = style.color;const contrast = contrastChecker.isContrastValid(backgroundColor, color);if (!contrast.passes) {this.addIssue('warning', 'low-contrast','颜色对比度不足', element);}});}// 检查键盘可访问性checkKeyboardAccess() {const interactive = document.querySelectorAll('a, button, input, select, textarea');interactive.forEach(element => {if (window.getComputedStyle(element).display === 'none' ||element.offsetParent === null) {return;}const tabIndex = element.tabIndex;if (tabIndex < 0) {this.addIssue('warning', 'keyboard-trap','元素不可通过键盘访问', element);}});}// 添加问题addIssue(severity, code, message, element) {this.issues.push({severity,code,message,element: element.outerHTML,location: this.getElementPath(element)});}// 获取元素路径getElementPath(element) {let path = [];while (element.parentElement) {let selector = element.tagName.toLowerCase();if (element.id) {selector += `#${element.id}`;}path.unshift(selector);element = element.parentElement;}return path.join(' > ');}// 生成报告generateReport() {return {timestamp: new Date().toISOString(),totalIssues: this.issues.length,issues: this.issues,summary: this.generateSummary()};}// 生成摘要generateSummary() {const counts = {error: 0,warning: 0};this.issues.forEach(issue => {counts[issue.severity]++;});return {errors: counts.error,warnings: counts.warning,score: this.calculateScore(counts)};}// 计算无障碍得分calculateScore(counts) {const total = counts.error + counts.warning;if (total === 0) return 100;const score = 100 - (counts.error * 5 + counts.warning * 2);return Math.max(0, Math.min(100, score));}
}

使用示例

// 初始化审计工具
const auditor = new AccessibilityAuditor();// 运行审计
async function runAudit() {const results = await auditor.audit();displayResults(results);
}// 显示结果
function displayResults(results) {const container = document.getElementById('audit-results');container.innerHTML = `<div class="audit-summary"><h2>无障碍审计结果</h2><p>得分: ${results.summary.score}</p><p>错误: ${results.summary.errors}</p><p>警告: ${results.summary.warnings}</p></div><div class="audit-issues">${results.issues.map(issue => `<div class="issue ${issue.severity}"><h3>${issue.message}</h3><code>${issue.location}</code><pre><code>${issue.element}</code></pre></div>`).join('')}</div>`;
}// 运行审计
runAudit();