Web Design in a Nutshell, 3rd Edition

In terms of markup and table structure, headers are placed in the tr element, the same as a td, as shown in this example.

    <table>
    <tr><th>Planet</th><th>Distance from Earth</th></tr>
    <tr><td>Venus</td><td>pretty darn far</td></tr>
    <tr><td>Neptune</td><td>ridiculously far</td></tr>
    </table>

User agents usually render the contents of table headers slightly differently than regular table cells (most often in bold, centered text); however, their appearance may easily be changed with style sheets.

The difference between th and td elements is not merely presentational, however. Table headers perform an important function in binding descriptive labels to table cells for non-visual browsers. They are discussed in more detail in the "Accessible Tables" section later in this chapter. Table header elements should not be used in layout tables.