简单的输入框value隐藏

<form action="" method="post" class="form_order">
<input id ="put_keyword" type="text" name="question_answer" class="order_like" value="Please input Question or Answer" onblur="if(this.value=='') this.value='Please input Question or Answer';" onfocus="if(this.value=='Please input Question or Answer') this.value='';" />
<input type="image"  src="shous.png" class="order_like_submit" />
</form>