Algo así
:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>ejem</title>
<script>
onload=function(){
var t=document.getElementById('ttt').getElementsByTagName('tr');
t[t.length-1].style.backgroundColor='red';
}
</script>
</head>
<body>
<table id="ttt" width="77" border="1" cellspacing="0" cellpadding="0">
<tr>
<td> </td>
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</tr>
<tr>
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</tr>
<tr>
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</tr>
<tr>
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</tr>
<tr>
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</tr>
<tr>
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</tr>
<tr>
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</tr>
<tr>
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</tr>
<tr>
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</tr>
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</tr>
</table>
</body>
</html>