checkbox validation

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
 <head>
 <script>
  function valthisform()
{
    var checkboxs=document.getElementsByName("c1");
    var okay=false;
    for(var i=0,l=checkboxs.length;i<l;i++)
    {
        if(checkboxs[i].checked)
        {
            okay=true;
        }
    }
    if(okay){
alert("Thank you for checking a checkbox");
}
    else{
 alert("Please check a checkbox");
return false;
}
}
</script>
 </head>

 <body>

Please select at least one Checkbox
<br>
<br>
<form name = "FC">
<input type = "checkbox" name = "c1" value = "c1"> C1 <br>
<input type = "checkbox" name = "c1" value = "c2"> C2 <br>
<input type = "checkbox" name = "c1" value = "c3"> C3 <br>
<input type = "checkbox" name = "c1" value = "c4"> C4 <br>
</form>
<br>
<br>

<input type = "submit" value = "Edit and Report" onClick = "return valthisform();">
 </body>
</html>