revision:
The colspan attribute can be used on the following elements: <td> and <th>. It allows the single table cell to span the width of more than one cell or column.
<td/th colspan="number">
number specifies the number of columns a cell should span. The number must be an integer. Note: colspan="0" tells the browser to span the cell to the last column of the column group (colgroup).
| Monthly Savings | |
|---|---|
| January | $100 | 
| January | $80 | 
                    <table>
                        <tr>
                        <th colspan="2">Monthly Savings</th>
                        </tr>
                        <tr>
                        <td>January</td>
                        <td>$100</td>
                        </tr>
                        <tr>
                        <td>January</td>
                        <td>$80</td>
                        </tr>
                    </table>
                    <style>
                        table, th, td {border: 1px solid black;}
                    </style>
                
            | Name | Expense | 
|---|---|
| Eddy | $10 | 
| Hilde | $8 | 
| Sum: $18 | |
                    <table>
                        <tr>
                            <th>Name</th>
                            <th>Expense</th>
                        </tr>
                        <tr>
                            <td>Eddy</td>
                            <td>$10</td>
                        </tr>
                        <tr>
                            <td>Hilde</td>
                            <td>$8</td>
                        </tr>
                        <tr>
                        <td colspan="2">Sum: $18</td>
                        </tr>
                    </table>
                    <style>
                        table, th, td {border: 1px solid black; border-collapse: collapse; 
                            padding: 6px; text-align:center;}
                    </style>