文本框默认值

 placeholder="Search W3School"

<input id="search" type="text" size="20" value="请输入关键字" onfocus="Onfocus()" onblur="Onblur()" />
<script type="text/javascript">
var Search=document.getElementById("search");
function Onfocus()
{
 if(Search.value=="请输入关键字")
 {
  Search.value="";
 }
}
function Onblur()
{
 if(Search.value=="")
 {
  Search.value="请输入关键字";
 }
}
</script>