$$("table").each(function(table){
  Selector.findChildElements(table, ["tr"])
    .findAll(function(row,i){ return i % 2 == 1; })
    .invoke("addClassName", "odd");
});

$$('#xxx tr:nth-child(odd)').each(function(el) {
    el.addClassName("odd");
});

$("tr:nth-child(odd)").addClass("odd");

$('#myid').css({
    backgroundColor: "#888",
    color: "gray",
    width: "100px"
})

var el = document.getElementById('myid');
el.style.backgroundColor = "#888";
el.style.color = "gray";
el.style.width = "100px";