mkdocs-material/material/partials/tabs-item.html

{% if nav_item.children and nav_item.children | length > 0 %}
  <li class="md-tabs__item">
    <a href="{{ nav_item.children[0].url }}" title="{{ nav_item.title }}" class="md-tabs__link {% if nav_item.active %} md-tabs__link--active {% endif %}">
      {{ nav_item.title }}
    </a>
  </li>
{% elif nav_item.url == base_url %}
  <li class="md-tabs__item">
    <a href="{{ nav_item.url }}" title="{{ nav_item.title }}" class="md-tabs__link {% for nav_item_ in nav %} {% if nav_item_.url == page.url and nav_item_.children | length == 0 %} md-tabs__link--active {% endif %} {% endfor %}">
      {{ nav_item.title }}
    </a>
  </li>
{% endif %}