Currently, Drupal required field is indicate by a red *, according to WCAG required, the page should indicate with word like, "* means required field" etc. It's possible to do in a better way, just indicate the require field with a red "require" key word.
This can be done in Theme template.php file with code,
$t = get_t();
$attributes = array(
'class' => 'form-required',
'title' => $t('This field is required.'),