Frontend A11y

// BAD: label has no connection to input — screen readers cannot associate them
<label>Email</label>
<input type="email" />

// GOOD: htmlFor matches input id
<label htmlFor="email">Email</label>
<input id="email" type="email" />

// BAD: visual-only asterisk conveys nothing to screen readers
<label htmlFor="email">Email *</label>
<input id="email" type="email" />

// GOOD: required enables native browser validation; aria-required signals it to screen readers
<label htmlFor="email">
  Email <span aria-hidden="true">*</span>
</label>
<input id="email" type="email" required aria-required="true" />

// BAD: error text exists visually but is not linked to the input
<input id="email" type="email" />
<span className="error">Invalid email address</span>

// GOOD: aria-describedby connects input to its error message
// aria-invalid signals the invalid state to screen readers
<input
  id="email"
  type="email"
  aria-describedby="email-error"
  aria-invalid={!!error}
/>
{error && (
  <span id="email-error" role="alert">
    {error}
  </span>
)}

interface LoginFormProps {
  onSubmit: (email: string, password: string) => void;
}

export function LoginForm({ onSubmit }: LoginFormProps) {
  const [email, setEmail] = useState('');
  const [password, setPassword] = useState('');
  const [errors, setErrors] = useState<{ email?: string; password?: string }>({});

  const handleSubmit = (e: React.FormEvent) => {
    e.preventDefault();
    const newErrors: typeof errors = {};
    if (!email) newErrors.email = 'Email is required';
    if (!password) newErrors.password = 'Password is required';
    if (Object.keys(newErrors).length) {
      setErrors(newErrors);
      return;
    }
    onSubmit(email, password);
  };

  return (
    <form onSubmit={handleSubmit} noValidate>
      <div>
        <label htmlFor="email">
          Email <span aria-hidden="true">*</span>
        </label>
        <input
          id="email"
          type="email"
          value={email}
          onChange={e => setEmail(e.target.value)}
          aria-required="true"
          aria-describedby={errors.email ? 'email-error' : undefined}
          aria-invalid={!!errors.email}
          autoComplete="email"
        />
        {errors.email && (
          <span id="email-error" role="alert">
            {errors.email}
          </span>
        )}
      </div>

      <div>
        <label htmlFor="password">
          Password <span aria-hidden="true">*</span>
        </label>
        <input
          id="password"
          type="password"
          value={password}
          onChange={e => setPassword(e.target.value)}
          aria-required="true"
          aria-describedby={errors.password ? 'password-error' : undefined}
          aria-invalid={!!errors.password}
          autoComplete="current-password"
        />
        {errors.password && (
          <span id="password-error" role="alert">
            {errors.password}
          </span>
        )}
      </div>

      <button type="submit">Log in</button>
    </form>
  );
}

// BAD: div has no role, no keyboard support, no accessible name
<div onClick={handleClick}>Submit</div>

// GOOD: button is focusable, activates on Enter/Space, announces as "button"
<button type="button" onClick={handleClick}>Submit</button>

// BAD: non-semantic navigation
<div onClick={() => navigate('/home')}>Home</div>

// GOOD: anchor supports right-click, middle-click, and keyboard navigation
<a href="/home">Home</a>

// BAD: heading hierarchy skipped (h1 to h4)
<h1>Dashboard</h1>
<h4>Recent Activity</h4>

// GOOD: sequential heading levels
<h1>Dashboard</h1>
<h2>Recent Activity</h2>

// aria-label: inline string label — use when no visible label text exists
<button aria-label="Close modal">
  <XIcon />
</button>

// aria-labelledby: references another element's text — use when a visible label exists
<section aria-labelledby="section-title">
  <h2 id="section-title">Recent Orders</h2>
  {/* content */}
</section>

// Provides supplementary description beyond the label
<button
  aria-describedby="delete-warning"
  onClick={handleDelete}
> Delete account
</button>
<p id="delete-warning">This action cannot be undone.</p>

// Use aria-live to announce content that updates without a page reload
// polite: waits for user to finish current action before announcing
// assertive: interrupts immediately — use only for urgent errors

export function StatusMessage({ message, isError }: { message: string; isError?: boolean }) {
  return (
    <div role="status" aria-live={isError ? 'assertive' : 'polite'} aria-atomic="true">
      {message}
    </div>
  );
}

export function Accordion({ title, children }: { title: string; children: React.ReactNode }) {
  const [isOpen, setIsOpen] = useState(false);
  const contentId = useId();

  return (
    <div>
      <button aria-expanded={isOpen} aria-controls={contentId} onClick={() => setIsOpen(prev => !prev)}>
        {title}
      </button>
      <div id={contentId} hidden={!isOpen}>
        {children}
      </div>
    </div>
  );
}

export function Dropdown({ options, onSelect }: { options: string[]; onSelect: (value: string) => void }) {
  const [isOpen, setIsOpen] = useState(false);
  const [activeIndex, setActiveIndex] = useState(0);
  const listId = useId();

  if (!options.length) return null;

  const handleKeyDown = (e: React.KeyboardEvent) => {
    switch (e.key) {
      case 'ArrowDown':
        e.preventDefault();
        setActiveIndex(i => Math.min(i + 1, options.length - 1));
        break;
      case 'ArrowUp':
        e.preventDefault();
        setActiveIndex(i => Math.max(i - 1, 0));
        break;
      case 'Enter':
      case ' ':
        e.preventDefault();
        if (isOpen) onSelect(options[activeIndex]);
        setIsOpen(prev => !prev);
        break;
      case 'Escape':
        setIsOpen(false);
        break;
    }
  };

  return (
    <div
      role="combobox"
      aria-expanded={isOpen}
      aria-haspopup="listbox"
      aria-controls={listId}
      tabIndex={0}
      onKeyDown={handleKeyDown}
      onClick={() => setIsOpen(prev => !prev)}
    >
      <span>{options[activeIndex]}</span>
      {isOpen && (
        <ul id={listId} role="listbox">
          {options.map((option, index) => (
            <li
              key={option}
              role="option"
              aria-selected={index === activeIndex}
              onClick={() => {
                onSelect(option);
                setIsOpen(false);
              }}
            >
              {option}
            </li>
          ))}
        </ul>
      )}
    </div>
  );
}

export function Modal({ isOpen, onClose, title, children }: { isOpen: boolean; onClose: () => void; title: string; children: React.ReactNode }) {
  const modalRef = useRef<HTMLDivElement>(null);
  const previousFocusRef = useRef<HTMLElement | null>(null);

  useEffect(() => {
    if (isOpen) {
      // Save currently focused element and move focus into modal
      previousFocusRef.current = document.activeElement as HTMLElement;
      modalRef.current?.focus();
    } else {
      // Restore focus to the element that opened the modal
      previousFocusRef.current?.focus();
    }
  }, [isOpen]);

  if (!isOpen) return null;

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

// BAD: decorative icon announced as unlabeled image
<img src="/icon.svg" />

// GOOD: decorative image hidden from screen readers
<img src="/decoration.png" alt="" aria-hidden="true" />

// GOOD: meaningful image with descriptive alt text
<img src="/chart.png" alt="Monthly revenue increased 23% from January to March" />

// GOOD: icon button with accessible label
<button aria-label="Delete item">
  <TrashIcon aria-hidden="true" />
</button>

export function useReducedMotion(): boolean {
  const [prefersReduced, setPrefersReduced] = useState(false);

  useEffect(() => {
    const mq = window.matchMedia('(prefers-reduced-motion: reduce)');
    setPrefersReduced(mq.matches);
    const handler = (e: MediaQueryListEvent) => setPrefersReduced(e.matches);
    mq.addEventListener('change', handler);
    return () => mq.removeEventListener('change', handler);
  }, []);

  return prefersReduced;
}

// Usage
export function AnimatedCard({ children }: { children: React.ReactNode }) {
  const reduceMotion = useReducedMotion();

  return (
    <div
      style={{
        transition: reduceMotion ? 'none' : 'transform 300ms ease'
      }}
    >
      {children}
    </div>
  );
}

// BAD: onClick on non-interactive element with no keyboard support
<div onClick={handleClick}>Click me</div>

// BAD: aria-label on a div that has no role
<div aria-label="Navigation">...</div>

// BAD: placeholder used as a substitute for label
<input placeholder="Enter your email" />

// BAD: positive tabIndex creates unpredictable tab order
<button tabIndex={3}>Submit</button>

// BAD: aria-hidden on a focusable element — keyboard users get trapped
<button aria-hidden="true">Open</button>

// BAD: role="button" on div without keyboard handler
<div role="button" onClick={handleClick}>Submit</div>
// Missing: tabIndex={0}, onKeyDown for Enter/Space