Markup for Form API elements of type #item is updated, removing 'for' attribute

Previously, a Form API element with '#type' => 'item' was rendered as

<div id="some-id">
  <label for="some-id">Item Label</label>
  ...
</div>

This is invalid HTML, since the <div> tag is not labelable. Reference: http://www.w3.org/html/wg/drafts/html/master/forms.html#the-label-element

The markup is now changed, replacing the <label> tag with a <span>. This tag has an id attribute, which is referenced from the aria-describedby attribute on the wrapping <div>:

<div aria-describedby="some-id">
  <span id="some-id">Item Label</span>
  ...
</div>

Theme changes

Themes with CSS rules that target the label will have to adjust their selectors.

Themes that override the form-element-label.html.twig template should review the changes to the version in the system module and consider adopting those changes.

Example

The first section on /admin/config/media/file-system comes from this render array in Drupal\system\Form\FileSystemForm:

    $form['file_public_path'] = [
      '#type' => 'item',
      '#title' => $this->t('Public file system path'),
      '#markup' => PublicStream::basePath(),
      '#description' => $this->t('A local file system path where public files will be stored. This directory must exist and be writable by Drupal. This directory must be relative to the Drupal installation directory and be accessible over the web. This must be changed in settings.php'),
    ];

Here is the markup generated for that form element when using the Default Admin theme:

Before

<div data-drupal-wrapper-selector="edit-file-public-path" id="edit-file-public-path" class="js-form-item form-item js-form-type-item form-type--item js-form-item-file-public-path form-item--file-public-path">
            <label for="edit-file-public-path" class="form-item__label">Public file system path</label>
              sites/default/files
              <div id="edit-file-public-path--description" class="form-item__description">
      A local file system path where public files will be stored. This directory must exist and be writable by Drupal. This directory must be relative to the Drupal installation directory and be accessible over the web. This must be changed in settings.php
    </div>
  </div>

After

<div data-drupal-wrapper-selector="edit-file-public-path" id="edit-file-public-path" aria-describedby="edit-file-public-path--label" class="js-form-item form-item js-form-type-item form-type--item js-form-item-file-public-path form-item--file-public-path">
            <span id="edit-file-public-path--label" class="form-item__label">Public file system path</span>
              sites/default/files
              <div id="edit-file-public-path--description" class="form-item__description">
      A local file system path where public files will be stored. This directory must exist and be writable by Drupal. This directory must be relative to the Drupal installation directory and be accessible over the web. This must be changed in settings.php
    </div>
  </div>