placeholder

var el = document.getElementById("target");
this.setPlaceholder(el,'#666','#000');
/**
  el: input 输入框；
  color: 提示文字颜色
  color2: 文本颜色
*/
function setPlaceholder(el,color,color2){
                //判断浏览器是否支持placeholder
	            function isPlaceHolder(){
	                var input = document.createElement("input");
	                return "placeholder" in input;
	            }
	            if(!isPlaceHolder()){
	                var txt = el.getAttribute('placeholder');
	                el.onfocus = function(){
	                    if(this.value == txt){
	                        this.value = '';
	                        this.style.color = color2 || '';
	                    }
	                }
	                el.onblur = function(){
	                    if(this.value == ''){
	                        this.value = txt;
	                        this.style.color = color;
	                    }
	                } 
	                el.value = txt;
	                el.style.color = color;
	            }
	        }