my-card-no-schema.twig

<div {{ attributes }}>
  <h2 class="component--my-card-no-schema__header">{{ header }}</h2>
  <div class="component--my-card-no-schema__body">
    {% block card_body %}
      Default contents for a card
    {% endblock %}
  </div>
</div>

File

core/modules/system/tests/modules/sdc_test_invalid/components/my-card-no-schema/my-card-no-schema.twig

<div {{ attributes }}>
  <h2 class="component--my-card-no-schema__header">{{ header }}</h2>
  <div class="component--my-card-no-schema__body">
    {% block card_body %}
      Default contents for a card
    {% endblock %}
  </div>
</div>