像文本框中的提示字符，鼠标点击进去就消失这样效果怎么用代码实现呢，谢谢

<input type='text' value='如:www.2345.com' style="color:#aaaaaa" onfocus="javascript:if (this.value=='如:www.2345.com'){this.value='';this.style.color='#000000';}" onblur="javascript:if (