js模仿html5 placeholder适应于不支持的浏览器

(function(){
//判断是否支持placeholder
function isPlaceholer(){
var input = document.createElement(input);
return "placeholder" in input;
}
//不支持的代码
if(!isPlaceholer()){
//创建一个类
function Placeholder(obj){
this.input = obj;
this.label = document.createElement(label);
this.label.innerHTML = obj.getAttribute(placeholder);
this.label.style.cssText = position:absolute; text-indent:4px;color:#999999; font-size:12px;;
if(obj.value != ){
this.label.style.display = none;
}
this.init();
}
Placeholder.prototype = {
//取位置
getxy : function(obj){
var left, top;
if(document.documentElement.getBoundingClientRect){
var html = document.documentElement,
body = document.body,
pos = obj.getBoundingClientRect(),
st = html.scrollTop || body.scrollTop,
sl = html.scrollLeft || body.scrollLeft,
ct = html.clientTop || body.clientTop,
cl = html.clientLeft || body.clientLeft;
left = pos.left + sl - cl;
top = pos.top + st - ct;
}
else{
while(obj){
left += obj.offsetLeft;
top += obj.offsetTop;
obj = obj.offsetParent;
}
}
return{
left: left,
top : top
}
},
//取宽高
getwh : function(obj){
return {
w : obj.offsetWidth,
h : obj.offsetHeight
}
},
//添加宽高值方法
setStyles : function(obj,styles){
for(var p in styles){
obj.style[p] = styles[p]+px;
}
},
init : function(){
var label = this.label,
input = this.input,
xy = this.getxy(input),
wh = this.getwh(input);
this.setStyles(label, {width:wh.w, height:wh.h, lineHeight:20, left:xy.left, top:xy.top});
document.body.appendChild(label);
label.onclick = function(){
this.style.display = "none";
input.focus();
}
input.onfocus = function(){
label.style.display = "none";
};
input.onblur = function(){
if(this.value == ""){
label.style.display = "block";
}
};
}
}
var inpColl = document.getElementsByTagName(input),
textColl = document.getElementsByTagName(textarea);
//html集合转化为数组
function toArray(coll){
for(var i = 0, a = [], len = coll.length; i < len; i++){
a[i] = coll[i];
}
return a;
}
var inpArr = toArray(inpColl),
textArr = toArray(textColl),
placeholderArr = inpArr.concat(textArr);
for (var i = 0; i < placeholderArr.length; i++){
if (placeholderArr[i].getAttribute(placeholder)){
new Placeholder(placeholderArr[i]);
}
}
}
})()

<div>
<input type="text" placeholder="提示1" /><br>
<textarea placeholder="提示2" /></textarea><br>
<input type="text" placeholder="提示3" /><br>
</div>

div,input,textarea{ margin:0; padding:0;}
div{width:400px; margin:100px auto 0;}
input,textarea{width:200px;height:20px; margin-top:5px;line-height:20px;border:1px #666666 solid; background-color:#fff; padding-left:2px;}
textarea{ height:60px; font-size:12px; resize:none;}