CSS mask-border Property - Playground Example

<!DOCTYPE html><html><head><style type="text/css">/* Layout */
:root {
  --border-size: 24;
}

body {
  font-family: system-ui, -apple-system, 'Segoe UI', Roboto, 'Helvetica Neue', Arial;
  background: #f3f6f9;
  margin: 0;
  padding: 40px 20px;
  display: flex;
  justify-content: center;
}

.demo {
  width: 100%;
  max-width: 720px;
}

/* Mask-border demo */
.masked-frame {
  --b: var(--border-size);
  background: linear-gradient(135deg, #ff9a9e 0%, #fad0c4 100%);
  color: #1b1b1b;
  padding: 20px 24px;
  border: calc(var(--b) * 1px) solid rgba(0,0,0,0.55);
  border-radius: 12px;
  box-shadow: 0 6px 18px rgba(19,30,43,0.08);
  line-height: 1.4;
  /* SVG data URI used as the mask-border source */
  -webkit-mask-border: url('data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0%200%20100%20100'><rect width='100' height='100' fill='black'/><rect x='14' y='14' width='72' height='72' fill='none' rx='12'/></svg>') var(--b) fill round;
  mask-border: url('data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0%200%20100%20100'><rect width='100' height='100' fill='black'/><rect x='14' y='14' width='72' height='72' fill='none' rx='12'/></svg>') var(--b) fill round;
  -webkit-mask-repeat: round;
  mask-repeat: round;
}

.masked-frame h2 {
  margin: 0 0 8px 0;
  font-size: 1.25rem;
}

.masked-frame p {
  margin: 0 0 14px 0;
  opacity: 0.95;
}

.btn {
  display: inline-block;
  padding: 8px 14px;
  background: rgba(27,27,27,0.9);
  color: #fff;
  text-decoration: none;
  border-radius: 8px;
  font-weight: 600;
}

/* Small screens */
@media (max-width: 420px) {
  .masked-frame {
    padding: 16px;
  }
}
</style></head><body><div class='demo'>
  <div class='masked-frame'>
    <h2>Decorative mask-border</h2>
    <p>This box uses an SVG data-uri as a mask-border to create a decorative frame.</p>
    <a class='btn' href='#'>Take action</a>
  </div>
</div></body></html>