Issue #3476649 by stefan.korn, supriyak: starterkit theme should use same...

 * Theme override for a fieldset element and its children.

 *

 * Available variables:

 * - attributes: HTML attributes for the fieldset element.

 * - errors: (optional) Any errors for this fieldset element, may not be set.

 * - required: Boolean indicating whether the fieldset element is required.

 * - legend: The legend element containing the following properties:

 *   - title: Title of the fieldset, intended for use as the text of the legend.

 *   - attributes: HTML attributes to apply to the legend.

 * - attributes: HTML attributes for the <fieldset> element.

 * - errors: (optional) Any errors for this <fieldset> element, may not be set.

 * - required: Boolean indicating whether the <fieldset> element is required.

 * - legend: The <legend> element containing the following properties:

 *   - title: Title of the <fieldset>, intended for use as the text

       of the <legend>.

 *   - attributes: HTML attributes to apply to the <legend> element.

 * - description: The description element containing the following properties:

 *   - content: The description content of the fieldset.

 *   - content: The description content of the <fieldset>.

 *   - attributes: HTML attributes to apply to the description container.

 * - children: The rendered child elements of the fieldset.

 * - prefix: The content to add before the fieldset children.

 * - suffix: The content to add after the fieldset children.

 * - description_display: Description display setting. It can have these values:

 *   - before: The description is output before the element.

 *   - after: The description is output after the element (default).

 *   - invisible: The description is output after the element, hidden visually

 *     but available to screen readers.

 * - children: The rendered child elements of the <fieldset>.

 * - prefix: The content to add before the <fieldset> children.

 * - suffix: The content to add after the <fieldset> children.

 *

 * @see template_preprocess_fieldset()

 */

    <span{{ legend_span.attributes.addClass(legend_span_classes) }}>{{ legend.title }}</span>

  </legend>

  <div class="fieldset-wrapper">

    {% if description_display == 'before' and description.content %}

      <div{{ description.attributes.addClass('description') }}>{{ description.content }}</div>

    {% endif %}

    {% if errors %}

      <div class="form-item--error-message">

        <strong>{{ errors }}</strong>

    {% if suffix %}

      <span class="field-suffix">{{ suffix }}</span>

    {% endif %}

    {% if description.content %}

    {% if description_display in ['after', 'invisible'] and description.content %}

      <div{{ description.attributes.addClass('description') }}>{{ description.content }}</div>

    {% endif %}

  </div>