placeholder 属性提供可描述输入字段预期值的提示信息（hint）。
该提示会在输入字段为空时显示，并会在字段获得焦点时消失。
注释：placeholder 属性适用于以下的 <input> 类型：text, search, url, telephone, email 以及 password。
demo例子
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>placeHolder</title>
</head>
<input type="text" id="fileElem" placeholder = "sssss" >
<input type="text" id="fileElem1" placeholder = "XXXXXXXXXXXXXXXXXX" >
<input type="text" id="fileElem" placeholder = "11111" >
<input type="text" id="fileElem" placeholder = "sss2222ss" >
<input type="text" id="fileElem" placeholder = "33333" >
<input type="text" id="fileElem" placeholder = "444444" >
<input type="text" id="fileElem" placeholder = "666666" >
<textarea placeholder="7777777777777777777">
</textarea>
<script>
var placeHolder = {
box:function(){
var obj = [];
var text = document.getElementsByTagName('input');
var textarea = document.getElementsByTagName('textarea');
for(var i=0;i < text.length;i++){
if(!!text[i].getAttribute('placeholder')){
obj.push(text[i]);
}else{
continue;
}
}
for(var j=0;j < textarea.length;j++){
if(!!textarea[j].getAttribute('placeholder')!=''){
obj.push(textarea[j]);
}else{
continue;
}
}
return obj;
},
IsSpport:function(){
var input = document.createElement('input');
return "placeholder" in input;
},
init:function(){
if(!this.IsSpport()){
var obj = this.box();
for(var i = 0;i < obj.length;i++){
obj[i].value = obj[i].getAttribute('placeholder');
obj[i].onfocus = function(e){
if(this.value == this.getAttribute('placeholder')){
this.value = '';
}
}
obj[i].onblur = function(){
if(this.value == ''){
this.value = this.getAttribute('placeholder');
}
}
}
}
}
}.init();
</script>
直接执行上诉js代码即可！谷歌，等高版本浏览器没效果！在IE低版本下测试！