    border: 1px solid;
      background: lightgrey;
    }

    subgrid {
      display: grid;
      grid: subgrid / subgrid;
      grid-column: 2 / span 2;
      grid-row: 1 / span 2;
      background: lightblue;
    }
  </style>
</head>

<body>
<grid>
  <item>1</item>
  <subgrid>
    <item>2</item>
    <item>3</item>
    <item>4</item>
  </subgrid>
  <item>5</item>
</grid>

<grid>
  <item style="width: 2ch">1</item>
  <subgrid>
    <item>2</item>
    <item>3</item>
    <item>4</item>
  </subgrid>
  <item>5