<div class="slider">
     <label>Name</label><input type="text" />
</div>
<div class="slider">
     <label>Street</label><input type="text" />
</div>
<div class="slider">
      <label>E-mail</label><input type="text" />
 </div>

  *{margin:0;padding:0;font-family: sans-serif,微软雅黑,Microsoft YaHei,Helvetica,Tahoma,StSun,宋体,SimSun;line-height: 1.5em;font-size: 14px;}
  .slider{position:relative;margin:0 0 10px 50px;}
  div.slider label{position:absolute;display:block;margin:1px 0 0 2px;padding:2px;}
  input[type="text"] { width:300px; border:1px solid #999; padding:5px; -moz-border-radius:4px;border-radius:4px;-webkit-border-radius:4px; }
  input[type="text"]:focus { border-color:#777; }

<script type="text/javascript">
        var labelSlider = {
            data: {
                tween: {
                    property: "left",
                    duration: 300
                },
                set: -50
            },
            start: function () {
                this.divContainers = $$("div.slider");
                for (var i = 0; i < this.divContainers.length; i++) {
                    this.divContainers[i].getElement("label").set("tween", this.data.tween);
                    this.divContainers[i].getElement("input").addEvents({
                        focus: this.left.bind(this, [i]),
                        blur: this.goback.bind(this, [i])
                    });
                }
            },
            left: function (at) {
                this.divContainers[at].getElement("label").get("tween").start(this.data.set);
            },
            goback: function (at) {
                if (this.divContainers[at].getElement("input").value == "") {
                    this.divContainers[at].getElement("label").get("tween").start(0);
                }
            }
        };
        labelSlider.start();
    </script>