Just put the CSS outside the PHP Tag like this
<body>
<table>
<tr><th>ID</th><th>hashtag</th></tr>
<?php
while($row = mysql_fetch_row($result))
{
echo "<tr onmouseover=\"hilite(this)\" onmouseout=\"lowlite(this)\"><td>$row[0]</td> <td>$row[1]</td></tr>\n";
}
?>
</table>
</body>