How to modify field group markup?
Implementing hook_field_group_pre_render_alter() seems the way to go, it's called just after field_group_field_group_pre_render() which calls field_group_pre_render_accordion_item() and receives the correct info.

It should be like this:

function mymodule_field_group_pre_render_alter(&amp;$element, $group, &amp;$form) {
  if ($group-&gt;format_type == 'accordion_item') {
    $element['#prefix'] = '&lt;div class="field-group-format-toggler ' . $group-&gt;format_type . ($group-&gt;collapsed ? '' : ' field-group-accordion-active') . '"&gt;&lt;a href="#"&gt;' . check_plain(t($group-&gt;label)) . '&lt;/a&gt;&lt;/div&gt;&lt;div class="field-group-format-wrapper ' . $group-&gt;classes . '"&gt;';
    if (!empty($group-&gt;description)) {
      $element['#prefix'] .= '&lt;div class="description"&gt;' . $group-&gt;description . '&lt;/div&gt;';
    }
  }
}


Remember to implement it in a module, as a hook_*_alter() implemented in a front-end theme may not be discovered while flushing cache via drush or in the back-end theme.