Menu callback; regular HTML page with <meta> element.
function openid_test_yadis_http_equiv() {
$element = array(
'#tag' => 'meta',
'#attributes' => array(
'http-equiv' => 'X-XRDS-Location',
'content' => url('openid-test/yadis/xrds', array(
'absolute' => TRUE,
)),
),
);
drupal_add_html_head($element, 'openid_test_yadis_http_equiv');
return t('This page includes a <meta equiv=...> element containing the URL of an XRDS document.');
}