What best practices should be followed when designing PHP forms to ensure consistent functionality across various browsers and devices?

When designing PHP forms, it is important to use HTML5 form elements and attributes to ensure compatibility across different browsers and devices. Additionally, validating user input on both the client-side and server-side can help prevent errors and improve user experience. Using CSS for styling and responsive design techniques can also ensure that the form displays correctly on various screen sizes.

<form action="submit.php" method="post">
  <label for="username">Username:</label>
  <input type="text" id="username" name="username" required>
  
  <label for="password">Password:</label>
  <input type="password" id="password" name="password" required>
  
  <input type="submit" value="Submit">
</form>