drupal 7 webform add classes to submit button
You can do this hook_form_alter(). As all the form element are available in hook_form_alter.

function mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id){
    if ($form_id == 'webform_client_form_YOUR_NODE_ID') {
        $form['actions']['submit']['#attributes']['class'][] = 'newclass';
    }
}