Client-Side Form Constraint API
Modern browsers enforce client-side validation natively without requiring JavaScript:
<!-- Regular expression pattern match for username -->
<input type="text" name="username" required pattern="[A-Za-z0-9_]{3,20}" title="Username must be 3-20 characters alphanumeric">
<!-- Numerical range boundaries -->
<input type="number" name="rating" min="1" max="5" step="0.5" value="5">