(Drupal 8) Add attribute aria-describedby to responsive image in Twig template.

mytheme.theme

<?php

/**
 * Implements hook_preprocess_node__HOOK().
 *
 * (a11y) Pass the media target_id to the template so we can use it to form
 * a unique id to target from aria-describedby attribute.
 */
function MYTHEME_preprocess_node__slide(array &$variables) {
  $node = $variables['node'];
  $image = $node->get('field_image_media')->getValue();
  $media_entity = $node->get('field_image_media')->first()->get('entity')->getTarget()->getValue();
  $variables['image_target_id'] = $media_entity->get('field_media_image')->target_id;
}

node--slide.html.twig

{#
/**
 * @file
 */
#}

{# (a11y) Taking a different approach so we can add `aria-describedby` attribute
  to the <img> tag. Note: `file_uri` function is provided by twig_tweak module. #}
{% set image_uri = node.field_image_media|file_uri %}
{%
  set responsiveImage = {
    '#theme': 'responsive_image',
    '#responsive_image_style_id': 'slide',
    '#uri': image_uri,
    '#attributes': { width: '1040', height: '585', alt: '', 'aria-describedby': 'description-' ~ image_target_id },
  }
%}

<article{{ attributes.addClass('node').removeAttribute('role') }}>

  {{ title_prefix }}
  {% if label and not page %}
    <h2{{ title_attributes }}>
      <a href="{{ url }}" rel="bookmark">{{ label }}</a>
    </h2>
  {% endif %}
  {{ title_suffix }}

  {% if display_submitted %}
    <footer>
      {{ author_picture }}
      <div{{ author_attributes }}>
        {% trans %}Submitted by {{ author_name }} on {{ date }}{% endtrans %}
        {{ metadata }}
      </div>
    </footer>
  {% endif %}

  <div{{ content_attributes }}>
    {# Render the responsive image. #}
    {{ responsiveImage }}
    
    {# A field has been added to the content type to contain a long
       description for screen readers. #}
    <div class="visually-hidden" id="description-{{ image_target_id }}">{{ node.field_image_description.value }}</div>
    
    {# Render the rest of the content without the fields above. #}
    {{ content|without('field_image_media', 'field_image_description') }}
  </div>

</article>