Barrierefreie Formulare bauen, die wirklich funktionieren

<!-- ❌ Label exists but doesn't help -->
<label for="field1">Field 1</label>
<input id="field1" type="text" />
 
<!-- ❌ Placeholder is not a label — disappears on input -->
<input type="email" placeholder="Enter your email" />
 
<!-- ✅ Clear label with format hint -->
<div class="field-group">
  <label for="phone">Phone number</label>
  <input
    id="phone"
    type="tel"
    aria-describedby="phone-hint"
    autocomplete="tel"
  />
  <span id="phone-hint" class="hint">
    Format: (555) 123-4567
  </span>
</div>
 
<!-- ✅ Required fields clearly indicated -->
<div class="field-group">
  <label for="email">
    Email address
    <span aria-hidden="true" class="required">*</span>
  </label>
  <input
    id="email"
    type="email"
    required
    aria-required="true"
    autocomplete="email"
    aria-describedby="email-hint"
  />
  <span id="email-hint" class="hint">
    We'll send your confirmation here
  </span>
</div>

<!-- ✅ Accessible error handling pattern -->
<form novalidate aria-label="Registration form">
  <!-- Error summary at top of form -->
  <div
    id="error-summary"
    role="alert"
    aria-live="assertive"
    class="error-summary"
    hidden
  >
    <h2>There are 2 problems with your submission</h2>
    <ul>
      <li><a href="#email">Enter a valid email address</a></li>
      <li><a href="#password">Password must be at least 8 characters</a></li>
    </ul>
  </div>
 
  <div class="field-group">
    <label for="email">Email address</label>
    <input
      id="email"
      type="email"
      required
      aria-required="true"
      aria-invalid="true"
      aria-describedby="email-error"
      autocomplete="email"
    />
    <span id="email-error" class="error" role="alert">
      Enter a valid email address
    </span>
  </div>
 
  <div class="field-group">
    <label for="password">Password</label>
    <input
      id="password"
      type="password"
      required
      aria-required="true"
      aria-invalid="true"
      aria-describedby="password-error password-requirements"
      autocomplete="new-password"
    />
    <span id="password-error" class="error" role="alert">
      Password must be at least 8 characters
    </span>
    <span id="password-requirements" class="hint">
      At least 8 characters with one uppercase letter and one number
    </span>
  </div>
</form>

// Form validation with accessible error handling
class AccessibleForm {
  private form: HTMLFormElement;
  private errorSummary: HTMLElement;
 
  constructor(form: HTMLFormElement) {
    this.form = form;
    this.errorSummary = form.querySelector('#error-summary')!;
    this.form.addEventListener('submit', (e) => this.handleSubmit(e));
  }
 
  private handleSubmit(e: Event): void {
    e.preventDefault();
    const errors = this.validate();
 
    if (errors.length > 0) {
      this.showErrors(errors);
    } else {
      this.clearErrors();
      this.submitForm();
    }
  }
 
  private showErrors(
    errors: Array<{ fieldId: string; message: string }>
  ): void {
    // Show error summary
    this.errorSummary.hidden = false;
    const list = this.errorSummary.querySelector('ul')!;
    list.innerHTML = '';
 
    for (const error of errors) {
      const li = document.createElement('li');
      const link = document.createElement('a');
      link.href = `#${error.fieldId}`;
      link.textContent = error.message;
      li.appendChild(link);
      list.appendChild(li);
 
      // Mark individual fields
      const field = document.getElementById(error.fieldId);
      if (field) {
        field.setAttribute('aria-invalid', 'true');
        const errorEl = document.getElementById(`${error.fieldId}-error`);
        if (errorEl) {
          errorEl.textContent = error.message;
          errorEl.hidden = false;
        }
      }
    }
 
    // Move focus to error summary so screen readers announce it
    this.errorSummary.focus();
  }
 
  private clearErrors(): void {
    this.errorSummary.hidden = true;
    this.form.querySelectorAll('[aria-invalid]').forEach((el) => {
      el.removeAttribute('aria-invalid');
    });
  }
}

/* ✅ Visible focus indicators — never remove outline without replacement */
/* ❌ Don't do this: *:focus { outline: none; } */
 
input:focus,
select:focus,
textarea:focus,
button:focus {
  outline: 3px solid #2563eb;
  outline-offset: 2px;
}
 
/* For browsers that support it, only show focus ring on keyboard nav */
input:focus:not(:focus-visible) {
  outline: none;
}
 
input:focus-visible {
  outline: 3px solid #2563eb;
  outline-offset: 2px;
}
 
/* Error state styling */
input[aria-invalid="true"] {
  border-color: #dc2626;
  border-width: 2px;
}
 
input[aria-invalid="true"]:focus {
  outline-color: #dc2626;
}
 
/* Skip link — first focusable element on the page */
.skip-link {
  position: absolute;
  top: -100%;
  left: 0;
  padding: 0.5rem 1rem;
  background: #1e293b;
  color: white;
  z-index: 100;
}
 
.skip-link:focus {
  top: 0;
}

// ✅ Custom dropdown that works with keyboard
class AccessibleSelect {
  private button: HTMLButtonElement;
  private listbox: HTMLUListElement;
  private options: HTMLLIElement[];
  private activeIndex: number = -1;
 
  handleKeyDown(event: KeyboardEvent): void {
    switch (event.key) {
      case 'ArrowDown':
        event.preventDefault();
        this.activeIndex = Math.min(
          this.activeIndex + 1,
          this.options.length - 1
        );
        this.updateActiveDescendant();
        break;
 
      case 'ArrowUp':
        event.preventDefault();
        this.activeIndex = Math.max(this.activeIndex - 1, 0);
        this.updateActiveDescendant();
        break;
 
      case 'Enter':
      case ' ':
        event.preventDefault();
        if (this.activeIndex >= 0) {
          this.selectOption(this.activeIndex);
        }
        break;
 
      case 'Escape':
        this.closeDropdown();
        this.button.focus();
        break;
 
      case 'Home':
        event.preventDefault();
        this.activeIndex = 0;
        this.updateActiveDescendant();
        break;
 
      case 'End':
        event.preventDefault();
        this.activeIndex = this.options.length - 1;
        this.updateActiveDescendant();
        break;
    }
  }
 
  private updateActiveDescendant(): void {
    const activeOption = this.options[this.activeIndex];
    this.listbox.setAttribute(
      'aria-activedescendant',
      activeOption.id
    );
    activeOption.scrollIntoView({ block: 'nearest' });
  }
}

<!-- ✅ Accessible multi-step form -->
<div role="group" aria-label="Checkout form">
  <!-- Step indicator -->
  <nav aria-label="Checkout progress">
    <ol class="steps">
      <li aria-current="step">
        <span class="step-number">1</span>
        <span>Shipping</span>
      </li>
      <li>
        <span class="step-number">2</span>
        <span>Payment</span>
      </li>
      <li>
        <span class="step-number">3</span>
        <span>Review</span>
      </li>
    </ol>
  </nav>
 
  <!-- Current step -->
  <section aria-label="Step 1: Shipping information">
    <h2>Shipping information</h2>
 
    <!-- Live region announces step changes -->
    <div
      aria-live="polite"
      aria-atomic="true"
      class="sr-only"
    >
      Step 1 of 3: Shipping information
    </div>
 
    <!-- Form fields for this step -->
    <div class="field-group">
      <label for="address">Street address</label>
      <input
        id="address"
        type="text"
        required
        autocomplete="street-address"
      />
    </div>
 
    <div class="button-group">
      <button type="button" disabled>Previous</button>
      <button type="button" onclick="nextStep()">
        Continue to payment
      </button>
    </div>
  </section>
</div>

## Accessibility testing checklist for forms
 
### Automated (run first)
- [ ] axe-core or Lighthouse audit: zero violations
- [ ] HTML validator: no markup errors
- [ ] All inputs have associated labels
- [ ] All images have alt text
 
### Keyboard testing (5 minutes)
- [ ] Tab through entire form — logical order?
- [ ] Every interactive element reachable via Tab
- [ ] Focus indicator visible on every element
- [ ] Custom controls work with Enter/Space
- [ ] Dropdown menus work with arrow keys
- [ ] Escape closes dropdowns/modals
- [ ] No keyboard traps (can always Tab out)
 
### Screen reader testing (15 minutes)
- [ ] NVDA or VoiceOver: navigate form by tab
- [ ] Each field announces: label, type, required state
- [ ] Error messages announced when they appear
- [ ] Error summary focused and read on submit
- [ ] Format hints read after label
- [ ] Multi-step progress communicated
 
### Visual testing
- [ ] Errors visible without relying on color alone
- [ ] Text meets 4.5:1 contrast ratio
- [ ] Form usable at 200% zoom
- [ ] Form works in high contrast mode