<div class="card">
  <div class="card__content">
    Lorem ipsum dolor sit amet consectetur adipisicing elit. Cupiditate porro vitae veniam esse accusantium quisquam dignissimos saepe soluta magni, fugiat officia blanditiis fugit vel quod reprehenderit aspernatur repellendus adipisci. Dolorem sit enim consequuntur voluptatum tempore odio adipisci deleniti dolor laborum, qui est asperiores labore, quaerat obcaecati error soluta hic ipsam.
  </div>
</div>

body {
  background-color: #aaa;
}

.card {
  width: 50%;
  margin: 0 auto;
  border: 64px solid transparent;
  border-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='256' height='256'%3E%3Cpath d='M0 64C0 33.8301 0 18.7452 9.37258 9.37258C18.7452 0 33.8301 0 64 0H192C222.17 0 237.255 0 246.627 9.37258C256 18.7452 256 33.8301 256 64V192C256 222.17 256 237.255 246.627 246.627C237.255 256 222.17 256 192 256H64C33.8301 256 18.7452 256 9.37258 246.627C0 237.255 0 222.17 0 192V64Z' fill='%23fff'/%3E%3C/svg%3E%0A") 64 fill;
  filter: drop-shadow(2px 2px 4px #0004)
}

.card__content {
  margin: -44px;
}