让IE支持placeholder属性

var JPlaceHolder = {
    //检测
    _check : function(){
        return 'placeholder' in document.createElement('input');
    },
    //初始化
    init : function(){
        if(!this._check()){
            this.fix();
        }
    },
    //修复
    fix : function(){
        jQuery(':input[placeholder]').each(function(index, element) {
            var self = $(this), txt = self.attr('placeholder');
            // 如果当前输入框中有值，不做处理
            if (self.val() != '') {
                // jquery each方法，return true = continue; return false = break;
                return true;
            }

            self.wrap($('<div></div>').css({position:'relative', zoom:'1', border:'none', background:'none', padding:'none', margin:'none'}));
            var pos = self.position(), h = self.outerHeight(true), paddingleft = self.css('padding-left');
            var holder = $('<span></span>').text(txt).css({position:'absolute', left:pos.left, top:'5px', height:h, lienHeight:h, paddingLeft:paddingleft, color:'#aaa'}).appendTo(self.parent());
            self.focusin(function(e) {
                holder.hide();
            }).focusout(function(e) {
                if(!self.val()){
                    holder.show();
                }
            });
            holder.click(function(e) {
                holder.hide();
                self.focus();
            });
        });
    }
};
//执行
jQuery(function(){
    JPlaceHolder.init();
});

<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>jQuery JPlaceholder Demo</title>
<script src="jquery-1.8.3.min.js"></script>
<script src="jquery.JPlaceholder.js"></script>
</head>
 
<body>
<form>
<div>
  <ul>
    <li>
      <input type="text" name="username" placeholder="用户名">
    </li>
    <li>
      <input type="password" name="username" placeholder="密码">
    </li>
    <li>
      <button type="button">登录</button>
    </li>
  </ul>
</div>
</form>
</body>
</html>