文本框内预先设定一个值，鼠标点击后该值消失，如果用户没有输入内容，鼠标离开，输入框又重新填充为预设值
在网页<body>区添加以下代码
<div>
<input size="8" maxlength="64" name="login_username" value="Username" onclick="if (this.value == 'Username') { this.value = ''; }" onblur="if (this.value == '') { this.value = 'Username'; }" type="text">
<input size="8" name="login_password" value="Password" onclick="if (this.value == 'Password') { this.value = ''; }" onblur="if (this.value == '') { this.value = 'Password'; }" type="password">
<input type="submit" accesskey="l" value="Login" class="formsubmit" name="login">
<br>
</div>
