;
  }
  .subgrid {
    display: grid;
    grid-column: 1 / -1;
    grid-row: 1 / -1;
    grid-template-columns: subgrid;
    grid-template-rows: subgrid;

    column-rule: blue solid 5px;
    row-rule: red solid 5px;

    column-rule-outset: 5px;
    column-rule-break: intersection;
  }
  .item {
    background: gray;
    opacity: 0.5;
  }
</style>
<body>
  <div class="grid-container">
    <div class="subgrid">
      <div class="item"></div>
      <div class="item"></div>
      <div class="item"></div>
    