placeholder 不支持ie8

//判断是否存在placeholder属性

                    function placeholder(el){
				function isPlaceHolder(){
					var textarea = document.createElement("textarea");
					return "placeholder" in textarea;
				}	
			
			//在不支持placeHolder属性的情况下
			if(isPlaceHolder() == false && !('placeholder' in document.createElement('textarea'))){
				$('textarea[placeholder]').each(function(){
					var that = $(this);
					text = that.attr('placeholder');
					
					if(that.val()===""){
						that.val(text);
					}
					
					//获得光标焦点后，输入框的内容清空
					that.focus(function(){
						if(that.val() == text){
							that.val("");
						}
					}).blur(function(){
						//光标焦点离开输入框时，给输入框添加提示内容
						if(that.val() === ""){
							that.val(text);
						}
					})
				})
			}
		}