9.2.2. Tag each row header and column header cell with <th>

In each data table, tag each row and column header cell with the <th> tag. In other words each cell that is used to understand the data shown in a table, must be tagged with <th>. 

<table>
    <caption>Average monthly temperatures of the 3 largest United States cities in 2012.</caption>
    <tr>
        <td>&nbsp;</td>
        <th>New York</th>
        <th>Boston</th>
        <th>Washington</th>
    </tr>
    <tr>
        <th>June</th>
        <td>22°C</td>
        <td>28°C</td>
        <td>26°C</td>
    </tr>
    <tr>
        <th>July</th>
        <td>24°C</td>
        <td>30°C</td>
        <td>28°C</td>
    </tr>
</table>

Average monthly temperatures of the 3 largest United States cities in 2012.
  New York Boston Washington
June 22°C 28°C 26°C
July 24°C 30°C 28°C