CSS

<div class="post">
  <div class="card">
    <h2>Card title</h2>
    <p>Card content</p>
  </div>
</div>

.post {
  container-type: inline-size;
}
 
/* Default heading styles for the card title */
.card h2 {
  font-size: 1em;
}
 
/* If the container is larger than 700px */
@container (width > 700px) {
  .card h2 {
    font-size: 2em;
  }
}

.post {
  container-type: inline-size;
  container-name: sidebar;
}
@container sidebar (width > 700px) {
  .card {
    font-size: 2em;
  }
}