$form['name'] => array(
  '#type' => 'textfield',
  '#title' => 'Prénom'
);

$form['name']['#attributes']['class'] = array('myClass');

$form['name'] => array(
  '#type' => 'textfield',
  '#title' => 'Prénom'
  '#prefix' => '<div class="myClass">',
  '#suffix' => '</div>'
);

.myClass label {
  text-transform: uppercase; /* just for example */
}

.form-item-field-foo label {
  /* CSS here */
}

function mymodule_form_alter(&$form, $form_state, $form_id) {
  switch ($form_id) {
    // Waterfall.
    case 'webform_client_form_16':
    case 'webform_client_form_51':
    case 'webform_client_form_64':
    case 'webform_client_form_78':
      $exclude = array('select', 'radios', 'checkboxes', 'managed_file');
      foreach ($form['submitted'] as $name => $component) {
        if (!in_array($component['#type'], $exclude) && $name != '#tree') {
          $form['submitted'][$name]['#prefix'] = '<span class= "label-invisible">';
          $form['submitted'][$name]['#suffix'] = '</span>';
          $form['submitted'][$name]['#attributes']['placeholder'] = $component['#title'];
        }
      }
      $form['#attached']['css'] = array(
        drupal_get_path('module', 'mymodule') . '/css/mymodule.form.css',
      );
    break;
  }
}

<span class="field-label"<?php print $title_attributes; ?>>
  <?php print $label; ?>:
</span>

<span class="<?php print $label; ?> field-label"<?php print $title_attributes; ?>>
  <?php print $label; ?>:
</span>

$form['some_element'] = array(
  '#type'          => 'textfield',
  '#title'         => '<span title="HELP!">'.t($subchildlabel).'</span>',
  '#default_value' => …
)
…
);

<label for="edit-m-vernacularname" class="inline"><span title="HELP!">Common name
</span> </label>

function your_module_preprocess_form_element(&$variables) {

  $element = $variables['element'];
  if (isset($element['#label_attributes'])) {
    $variables['label']['#attributes'] = array_merge(
      $variables['attributes'],
      $element['#label_attributes']
    );
  }
}

$form['some_field'] = [
  [...]
  '#label_attributes' => [
    'some_attr' => 'some_value',
  ]
]