<table>
  <tr>
    <th></th>
    <th>1</th>
    <th>2</th>
    <th>3</th>
  </tr>
  <tr>
    <th>1</th>
    <td>1</td>
    <td>2</td>
    <td>3</td>
  </tr>
  <tr>
    <th>2</th>
    <td>2</td>
    <td>4</td>
    <td>6</td>
  </tr>
  <tr>
    <th>3</th>
    <td>3</td>
    <td>6</td>
    <td>9</td>
  </tr>
</table>
body {
  width: 600px;
  margin: 1.5em auto;
}
table {
  border: 1px solid #ccc;
  border-collapse: collapse;
}
th, td {
  padding: .75em 4em;
  border: 1px solid #ccc;
}
th {
  background: #f4f4f4;
}
th:empty,
td:empty {
  background-image: linear-gradient(to top right, transparent, transparent 49%, #aaa 51%, transparent 51%, transparent 100%);
}

External CSS

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External JavaScript

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