How to make first row and all columns transparent in css?

Question

How to write code for this using HTML and CSS.

Please note that first row and columns in first row are transparent

Answer 1

table {
    border-collapse: collapse;
}

th {
    height: 6rem;
    width: 6rem;
    text-align: left;
}

.background {
    background-color: lightgrey;
}

tr {
    height: 6rem;
    width: 6rem;
}

td {
    border: 1px solid lightgrey;
    border-collapse: collapse;
}

<table>
      <tr>
        <th>A</th>
        <th>B</th>
        <th>C</th>
        <th>D</th>
      </tr>
      <tr class="background">
        <td>9</td>
        <td>9</td>
        <td>9</td>
        <td>9</td>
      </tr>
      <tr>
        <td>4</td>
        <td>4</td>
        <td>4</td>
        <td>4</td>
      </tr>
     </table>