Drupal 7: Add HTML5 required attribute to all form elements with '#required' => TRUE.

Drupal7_html5_required.php

<?php

/**
 * This goes in YOUR_MODULE.module file.
 */

function YOUR_MODULE_form_alter(&$form, &$form_state, $form_id) {
  // Always add the html5 required attribute for required elements.
  _YOUR_MODULE_html5_required($form);
}

function _YOUR_MODULE_html5_required($element) {
  if (is_array($element)) {
    if ((isset($element['#type']) && $element['#type'] !== 'checkboxes') && isset($element['#required']) && $element['#required']) {
      $element['#attributes']['required'] = '';
    }
    else {
      foreach (element_get_visible_children($element) as $key) {
        _YOUR_MODULE_html5_required($element[$key]);
      }
    }
  }
}

function _YOUR_MODULE_html5_required($element) should be function _YOUR_MODULE_html5_required(&$element) .