@layer blocks {
  /* The red margin rule, drawn at the notebook composition's rule edge. */
  .margin-rule::before {
    content: "";
    position: absolute;
    inset-block: 0;
    inset-inline-start: var(--notebook-rule-at);
    inline-size: var(--stroke-m);
    background: var(--color-accent-border);
  }
}