my-card.twig

<div {{ attributes }}>
  <h2 class="component--my-card__header">{{ header }}</h2>
  <div class="component--my-card--replaced__body">
    {% block card_body %}
      Default contents for a card
    {% endblock %}
  </div>
</div>

File

core/modules/system/tests/themes/sdc_theme_test_enforce_schema/components/my-card/my-card.twig

<div {{ attributes }}>
  <h2 class="component--my-card__header">{{ header }}</h2>
  <div class="component--my-card--replaced__body">
    {% block card_body %}
      Default contents for a card
    {% endblock %}
  </div>
</div>