<form action="{{ cart_url }}" method="post" data-product-id="{{ product.id }}">
  {% if product.show_variants? %}
    <select name="variant_id" data-product-variants>
      {% for variant in product.variants %}
        <option value="{{ variant.id }}">{{ variant.title | escape }}</option>
      {% endfor %}
    </select>
  {% else %}
    <input type="hidden" name="variant_id" value="{{product.variants.first.id}}" >
  {% endif %}
  <div data-quantity>
    <input type="text" name="quantity" value="1" />
    <span data-quantity-change="-1">-</span>
    <span data-quantity-change="1">+</span>
  </div>
  <button data-item-add class="btn">
    Добавить в корзину
  </button>
</form>