功能:
1、按回车键,光标跳转到下一文本框中
2、验证表单,若用户名或密码为空,后面出现提示信息
<!DOCTYPE html>
<html>
<head lang="en">
<meta charset="UTF-8">
<title></title>
<style type="text/css">
span {
color: transparent;
}
</style>
</head>
<body>
<form action="javascript: alert('提交成功');">
用户名:<input id="username" type="text" onkeyup="usernameValidate(this)" /><span id="usernameWarn"></span><br/><br/>
邮箱:<input id="mail" type="text" onkeyup="mailValidate(this)" /><span id="mailWarn"></span><br/><br/>
职业:<input id="job" type="text" onkeyup="jobValidate(this)" /><span id="jobWarn"></span><br/><br/>
</form>
<button onclick="register()">注册</button>
</body>
<script type="text/javascript">
function usernameValidate(thisObj