Setting a Tab Index in a Twig Template

facet-checkboxes.twig

<section class="m-facet-checkboxes block-facet--checkbox">
  <h2 class="block-title">{{ facet_checkboxes.block_title }}</h2>
  <ul class="js-facets-checkbox-links">
    {% for item in facet_checkboxes.checkboxes %}
      {% set counter = ( counter | default(0) ) + 1 %}
      <li class="facet-item">
        <input type="checkbox" class="facets-checkbox" tabindex="{{ counter }}" id="{{ item.id }}">
        <label for="{{ item.id }}">
          <span class="facet-item__value">{{ item.label }}</span>
          {% if item.count %}
            <span class="facet-item__count">({{ item.count }})</span>
          {% endif %}
        </label>
      </li>
    {% endfor %}
  </ul>
  <a href="#" class="facets-soft-limit-link">Show more</a>
</section>