HTML Tabella complessa

<table border="1">
    <tr>
        <th>colonna 1</th>
        <th>colonna 2</th>
        <th>colonna 3</th>
    </tr>
    <tr>
        <td>riga 2, cella 1</td>
        <td colspan="2">riga 2, cella 2 unita a riga 2, cella 3</td>
    </tr>
    <tr>
        <td rowspan="2">riga 3, cella 1 unita a riga 4, cella 1</td>
        <td>riga 3, cella 2</td>
        <td>riga 3, cella 3</td>
    </tr>
    <tr>
        <td>riga 4, cella 2</td>
        <td>Riva 4, cella 3</td>
    </tr>
</table>