Drupal : Add AJAX validation and submit to a regular form

MY_MODULE.module

<?php

function MY_MODULE_form_alter(&$form, &$form_state, $form_id) 
{
  $form['#prefix'] = '<div id="formwrapper">';
  $form['#suffix'] = '</div>';
  // the submit button
  $form['save']['#ajax'] = array(
    'callback' => 'MY_MODULE_form_ajax_submit',
    'wrapper' => 'formwrapper',
    'method' => 'replace',
    'effect' => 'fade',
  );
}

function MY_MODULE_form_ajax_submit($form, &$form_state) 
{
  // validate the form : ONLY IF NECESSARY
  // drupal_validate_form('mymodule_form_id', $form, $form_state);
  // if there are errors, return the form to display the error messages
  if (form_get_errors()) {
    $form_state['rebuild'] = TRUE;
    return $form;
  }
  // process the form : ONLY IF NECESSARY
  //mymodule_form_id_submit($form, $form_state);
  $output = array('#markup' => '<div class="messages status">'.t('Your request has been processed').'</div>');
  
  // return the confirmation message
  return $output;
}

I have two notes which may be useful:

1. drupal_validate_form method is called automatically if the form['#validate'] is populated
2. add 'event' => 'click' to the $form['save']['#ajax'] array, it enables submitting by pressing ENTER key and it also fixes autocompletion issues