Q:

add html to fieldset

I try to customize fieldset ABC that are inside drupal commerce checkout page. 
Markup looks like this
<fieldset>
 <legend>ABC</legend>
 <div class="fieldset-wrapper">
  <div class="form-item"></div>
  <div class="form-item"></div>
 </div>
</fieldset>

I would like to put some additional code directly inside fieldset and then also some other html directly inside form-item elements.  
Question> What should I do to achieve that?  I found this page https://api.drupal.org/api/drupal/includes!form.inc/function/theme_fieldset/7 would it be of any help?  Thank you. 
images

A:

Try to change it using hook_form_FORM_ID_alter(). 
// Replace FORM_ID with commerce checkout form id
function hook_form_FORM_ID_alter($form, &$form_state, $form_id) {
  $form['fieldset_item']['mymarkup'] = array(
    '#type' => 'item',
    '#markup' => 'My Markup Here.', 
  );
}

