<table>
  <tr>
    <th>1</th>
    <th>2</th>
    <th>3</th>
    <th>4</th>
    <th>5</th>
  </tr>
  <tr>
    <th>2</th>
    <td>4</td>
    <td>6</td>
    <td>8</td>
    <td>10</td>
  </tr>
  <tr>
    <th>3</th>
    <td>6</td>
    <td>9</td>
    <td>12</td>
    <td>15</td>
  </tr>
  <tr>
    <th>4</th>
    <td>8</td>
    <td>12</td>
    <td>16</td>
    <td>20</td>
  </tr>
  <tr>
    <th>5</th>
    <td>10</td>
    <td>15</td>
    <td>20</td>
    <td>25</td>
  </tr>
</table>

td, th {
  width: 4rem;
  height: 2rem;
  border: 1px solid #ccc;
  text-align: center;
}
th {
  background: lightblue;
  border-color: white;
}
body {
  padding: 1rem;
}