Use aria-describedby to link descriptions to input forms.

The trick here is to do the following in theme_webform_element and theme_element:

if (!empty($element['#description'])) {
    $decription_attributes = array('class' => array('description'));
    if ($element['#id']) {
      $decription_attributes['id'] = $element['#id'] . '-description';
    }
    $output .= '<div' . drupal_attributes($decription_attributes) . '>' . $element['#description'] . "</div>\n";
  }

But then you also need to use preprocess functions in your theme/module to add in the aria-describedby attribute to the form elements. There are lots, some are webform specific theme functions and others are core:

/**
 * Preprocess textareas to add in aria attributes.
 */
function yourtheme_preprocess_textarea(&$variables) {
  yourtheme_add_aria_attributes($variables);
}

/**
 * Preprocess textfield to add in aria attributes.
 */
function yourtheme_preprocess_textfield(&$variables) {
  yourtheme_add_aria_attributes($variables);
}

/**
 * Preprocess radios to add in aria attributes.
 */
function yourtheme_preprocess_radios(&$variables) {
  yourtheme_add_aria_attributes($variables);
}

/**
 * Preprocess checkboxes to add in aria attributes.
 */
function yourtheme_preprocess_checkboxes(&$variables) {
  yourtheme_add_aria_attributes($variables);
}

/**
 * Preprocess select to add in aria attributes.
 */
function yourtheme_preprocess_select(&$variables) {
  yourtheme_add_aria_attributes($variables);
}

/**
 * Preprocess date to add in aria attributes.
 */
function yourtheme_preprocess_date(&$variables) {
  yourtheme_add_aria_attributes($variables);
}

/**
 * Preprocess webform_email to add in aria attributes.
 */
function yourtheme_preprocess_webform_email(&$variables) {
  yourtheme_add_aria_attributes($variables);
}

/**
 * Preprocess webform_number to add in aria attributes.
 */
function yourtheme_preprocess_webform_number(&$variables) {
  yourtheme_add_aria_attributes($variables);
}

function yourtheme_add_aria_attributes(&$variables) {
  if (!empty($variables['element']['#description'])) {
    $variables['element']['#attributes']['aria-describedby'] = $variables['element']['#id'] . '-description';
  }
}

.... and I'm sure there are plenty more.

An example of the output would be the following:

<input id="edit-submitted-email-address" class="email form-text form-email required" type="email" size="60" name="submitted[email_address]" aria-describedby="edit-submitted-email-address-description">
<div id="edit-submitted-email-address-description" class="description">This will only be used to follow-up with you if there are additional questions.</div>

it also would be cool to integrate following ARIA-Features in the webform module:
- aria-reqired for requred fields
- aria-labelledby for errors

There is an attempt to do that here:
https://www.drupal.org/project/webform_aria

Should i make an own ticket for that?

@mgifford, @maki3000 -- I support someone writing a patch for ARIA support, provided people test it and mark it RTBC.

Here is a patch that has the desired effect. However, I really don't like the way that it does it. I don't like how I had to use the "--WEBFORM-PLACEHOLDER--", however, I couldn't find a place where the component had an #id, but had not yet been rendered into #children. Perhaps this happens inside Drupal core. I would appreciate any suggestions for making this better.

If the "--WEBFORM-PLACEHOLDER--" approach is to be used, a regex could allow it to deal with other elements setting @aria-describedby.

@maki3000, the other ARIA-related things you mention should each get their own tickets.

The maintainers have decided to defer all aria issues to D8. They are marked postponed because they will be looked at after the D8 port is functional.

Closing to clear out the old Webform 8.x-4.x branch. See #2827845: [roadmap] YAML Form 8.x-1.x to Webform 8.x-5.x.