CodePen Embed - gOpvgJv

<table class="table-grid">
  <tr>
    <td>Months

      <table class="table-grid">
        <tr>
          <td>Jan</td>
          <td>Feb</td>
          <td>Mar</td>
          <td>Apr</td>
        </tr>
      </table>

    </td>
  </tr>
</table>

.table-grid {
  width: 100%;
  border-collapse: collapse;
}

.table-grid>tbody>tr>th {
  padding: 0;
}

.table-grid>thead>tr>th,
.table-grid>tbody>tr>th,
.table-grid>tfoot>tr>th,
.table-grid>thead>tr>td,
.table-grid>tbody>tr>td,
.table-grid>tfoot>tr>td {
  border: 1px solid red;
}

.table-grid .table-grid td {
  border-top: 1px solid red;
  border-right: 1px solid red;
  border-bottom: 0;
  border-left: 0;
}

.table-grid .table-grid td:last-child {
  border-right: 0;
}

This Pen is owned by Vikas Lalwani on CodePen.

See more by @vikas-lalwani on CodePen

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.