Adding an HTML 5 placeholder attribute to a webform component in a theme

An HTML 5 placeholder attribute is added to a theme by using a preprocess hook. This example is for an e-mail component:

<?php
function THEMENAME_preprocess_webform_email(&$variables) {
  $variables['element']['#attributes']['placeholder'] = t('Email Address');
}
?>

For anyone using an older or outdated version of Webform, it's worth noting that HTML5 placeholder support was added in the 7.x-4.x branch (June 2013) and is now a native, built-in option when adding or editing a form component.