The Key HTML5 Interview Questions that Matter the Most

// Feature detection and fallback validation
function validateForm(form) {
  if (!form.checkValidity || !form.checkValidity()) {
    // Custom validation for older browsers
    const inputs = form.querySelectorAll('[required]');
    inputs.forEach(input => {
      if (!input.value.trim()) {
        showErrorMessage(input, 'This field is required');
        return false;
      }
    });
  }
  return true;
}

<video controls>
  <source src="video.webm" type="video/webm">
  <source src="video.mp4" type="video/mp4">
  <source src="video.ogv" type="video/ogg">
  <p>Your browser doesn't support HTML5 video. 
     <a href="video.mp4">Download the video</a>.</p>
</video>

// Debug service worker status
navigator.serviceWorker.ready.then(registration => {
  console.log('SW registered:', registration);
  console.log('SW active:', registration.active);
});
// Check cache contents
caches.keys().then(names => {
  names.forEach(name => {
    caches.open(name).then(cache => {
      cache.keys().then(keys => {
        console.log(`Cache ${name}:`, keys.map(k => k.url));
      });
    });
  });
});

// Object pooling for mobile performance
class ObjectPool {
  constructor(createFn, resetFn, initialSize = 10) {
    this.createFn = createFn;
    this.resetFn = resetFn;
    this.pool = [];
    
    for (let i = 0; i < initialSize; i++) {
      this.pool.push(this.createFn());
    }
  }
  
  get() {
    return this.pool.pop() || this.createFn();
  }
  
  release(obj) {
    this.resetFn(obj);
    this.pool.push(obj);
  }
}
// Efficient mobile rendering
function renderMobile(ctx, objects) {
  // Batch similar operations
  ctx.fillStyle = '#FF0000';
  objects.forEach(obj => {
    if (obj.visible) ctx.fillRect(obj.x, obj.y, obj.width, obj.height);
  });
}

<form novalidate aria-live="polite">
  <fieldset>
    <legend>Account Information</legend>
    
    <div class="form-group">
      <label for="username">
        Username <span aria-label="required">*</span>
      </label>
      <input type="text" 
             id="username" 
             name="username" 
             required 
             aria-describedby="username-error username-help"
             autocomplete="username">
      <div id="username-help" class="help-text">
        Must be 3-20 characters long
      </div>
      <div id="username-error" class="error-message" role="alert"></div>
    </div>
    
    <div class="form-group">
      <fieldset>
        <legend>Preferred Contact Method</legend>
        <label>
          <input type="radio" name="contact" value="email" checked>
          Email
        </label>
        <label>
          <input type="radio" name="contact" value="phone">
          Phone
        </label>
      </fieldset>
    </div>
  </fieldset>
  
  <button type="submit" aria-describedby="submit-help">
    Create Account
  </button>
  <div id="submit-help">
    By creating an account, you agree to our terms of service
  </div>
</form>