$('#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>