$('#login').click(function(){
$('#username').focus();
if ($(":test").val().length == 0){
$('#username')
}else if ($(':password').val().length == 0){
}
});
<p id="errormessage"></p>
<input id="username" type="test" name="username"><br>
<input type="password" name="password"><br>
<button id="login">登陆</button>