<body>
<table id="tbl">
<tr><td>1</td></tr>
<tr><td>2</td></tr>
<tr><td>3</td></tr>
<tr><td>4</td></tr>
<tr><td>5</td></tr>
<tr><td>6</td></tr>
<tr><td>7</td></tr>
<tr><td>8</td></tr>
<tr><td>9</td></tr>
<tr><td>10</td></tr>
</table>
</body><script type="text/javascript">
window.onload = function() {
var tbl = document.getElementById("tbl");
rows = tbl.getElementsByTagName("tr");
for (i = 0; i < rows.length; i++) {
var j = parseInt(i / 3);
if (j % 2 == 0)
rows[i].style.backgroundColor = "#f00";
else
rows[i].style.backgroundColor = "#0f0";
}
}
</script>
