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！