Tables which contain data (as opposed to layout tables) should contain th elements to mark headers for screen readers and enhance the structure of the document.

Example

Wrong

<table><tr><td>Header One</td><td>Header Two</td></tr><tr><td>1.30</td><td>4.50</td></tr></table>

Right

<table><tr><th>Header One</th><th>Header Two</th></tr><tr><td>1.30</td><td>4.50</td></tr></table>