How to change the user registration form markup
You can do that by implementing hook_form_alter() in your custom module:

For ex:

function MODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if($form_id == 'user_register_form') {
    //dpm($form);
    $form['account']['name']['#description'] = t('Your Custom Text');
  }
}


To add Markup you should look into theme_form_element.