$("ul :first") //伪对象选择符:取第一个ul元素。The :first pseudo-class is equivalent to:eq( 0 ). It could also be written as :lt( 1 ). While this matches only a single element, :first-child can match more than one: One for each parent.
$("#div1").first() //As you are looking through your code, you notice that someone else is selecting the last vacation with: $("#div1 :last"). You look at this and you think,"Traversal would make this way faster!".这就是使用DOM和使用CSS选择器的区别。