Center an LoginForm using declarative UI.

Hi,
I’m just testing the Vaadin 8.3.3 using the declarative UI design.
But I’m not able to center the an LoginForm.

LoginUI.html

<!DOCTYPE html>
<html>
<body>
 	<v-vertical-layout size-full>
		<v-login-form _id="loginForm" :middle :center/>
	</v-vertical-layout>
</body>
</html>

I have tried using horizontal and CSS layout. All with the same result.

What is wrong here?