效果演示：
代码：
<!DOCTYPE html >
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>onfocus和onblur的使用</title>
</head>
<body>
<input id="box" type="text" value="请输入Email"/>
<script type="text/javascript">
var input1=document.getElementById("box");
input1.onfocus=function(){
if(input1.value=="请输入Email"){
input1.value="";
}
}
input1.onblur=function(){
if(input1.value==""){
input1.value="请输入Email";
}
}
</script>
</body>
</html>